build: use the main branch (#66)

Co-authored-by: BenWhitehead <[email protected]>

README.md

@@ -11,11 +11,11 @@ consuming the conformance tests, this can be minimized by each
 API abiding by the same conventions:
 
 - Each API's tests are entirely within a directory of the form
-`{api-name}/{api-version}`. Example: [storage/v1](https://github.com/googleapis/conformance-tests/blob/master/storage/v1)
+`{api-name}/{api-version}`. Example: [storage/v1](https://github.com/googleapis/conformance-tests/blob/main/storage/v1)
 - A .proto file specifies the format of the test cases. Example:
-[Firestore tests](https://github.com/googleapis/conformance-tests/blob/master/firestore/v1/proto/google/cloud/conformance/firestore/v1/tests.proto)
+[Firestore tests](https://github.com/googleapis/conformance-tests/blob/main/firestore/v1/proto/google/cloud/conformance/firestore/v1/tests.proto)
 - Test cases are specified as a set of inputs and expected outputs, and are
-defined in .json files. Example: [Firestore test case](https://github.com/googleapis/conformance-tests/blob/master/firestore/v1/testcase/create-nosplit.json)
+defined in .json files. Example: [Firestore test case](https://github.com/googleapis/conformance-tests/blob/main/firestore/v1/testcase/create-nosplit.json)
 
 ### Proto conventions
 
@@ -26,15 +26,15 @@ which should start with `google.cloud.conformance` and reflect the
 API name and version. For example, the Firestore v1 test protos are
 in the `google.cloud.conformance.firestore.v1` proto package, and
 are in a directory of
-[firestore/v1/proto/google/cloud/conformance/firestore/v1](https://github.com/googleapis/conformance-tests/tree/master/firestore/v1/proto/google/cloud/conformance/firestore/v1).
+[firestore/v1/proto/google/cloud/conformance/firestore/v1](https://github.com/googleapis/conformance-tests/tree/main/firestore/v1/proto/google/cloud/conformance/firestore/v1).
 
 Note: while the directory structure here ends up being very deep, it
 means we follow the proto convention, which makes importing protos
 from each other simpler.
 
 The test protos may refer to protos within the production API. There
 is no versioning applied to these imports; it's assumed that it's
-always sufficient to take the head of the master branch in
+always sufficient to take the head of the main branch in
 https://github.com/googleapis/googleapis.
 
 Each API should have a `TestFile` message containing repeated fields
@@ -74,7 +74,7 @@ Each set of tests should come with a validator to ensure that the
 JSON files conform with the proto specification. The validator
 should be written in Go in the API-specific directory, in a file
 called `validator.go`. This should be invoked from the
-[.kokoro/validator.sh](https://github.com/googleapis/conformance-tests/tree/master/.kokoro/validator.sh)
+[.kokoro/validator.sh](https://github.com/googleapis/conformance-tests/tree/main/.kokoro/validator.sh)
 script which is executed as part of continuous integration.
 
 In most cases you don't need to know Go in order to write a