add a horizon endpoint doc

docs/build/guides/testing/unit-tests.mdx

@@ -25,6 +25,7 @@ fn test() {
     assert_eq!(client.increment(), 3);
 }
 ```
+
 Ref: https://github.com/stellar/soroban-examples/blob/main/increment/src/test.rs
 
 :::tip
@@ -42,6 +43,7 @@ The Env created at the beginning of the test is not a simulation of the Soroban
 It's a simple test, but it's a complete test. There's a full environment setup, used, and torn down in the test, and it happens fast. The Rust test harness runs all the tests for a contract in parallel and each will have its own isolated contract environment.
 
 Most tests, even integration tests and fuzz tests, will look very similar to this unit test. They'll do four things:
+
 1. Create an environment, the `Env`.
 2. Register the contract(s) to be tested.
 3. Invoke functions using a client.