chore: fix file reference (#1694)

README.md

@@ -11,7 +11,7 @@ The Cloud SQL Connector for Java is a library that provides IAM-based authorizat
 For usage information specific to your database engine and driver, see the pages below:
 
 * [Connecting to Cloud SQL using JDBC](docs/jdbc.md)
-* [Connecting to Cloud SQL using R2DBC](docs/r2dbc)
+* [Connecting to Cloud SQL using R2DBC](docs/r2dbc.md)
 
 For examples of this library being used in the context of an application, check out the sample applications located 
 [here](https://github.com/GoogleCloudPlatform/java-docs-samples/tree/master/cloud-sql).