Change Registr to Register to fix typo (#2545)

Co-authored-by: Ronald Bosma <[email protected]>

docs/Extend/Value-Retriever.md

@@ -86,7 +86,7 @@ public class ShirtColorValueRetriever : IValueRetriever
 
 ## Registering Custom ValueRetrievers
 
-Before you can utilize a custom `ValueRetriever`, you'll need to register it. We recommend doing this prior to a test run using the `[BeforeTestRun]` attribute and `Service.Instance.ValueRetrievers.Registr()`. For example:
+Before you can utilize a custom `ValueRetriever`, you'll need to register it. We recommend doing this prior to a test run using the `[BeforeTestRun]` attribute and `Service.Instance.ValueRetrievers.Register()`. For example:
 
 ``` csharp
 [Binding]