RELEASING.md: Include version number in release branch name

This makes it easier to manage multiple patch releases in-flight
simultaneously and improves the default PR description.

Author: ejona86

RELEASING.md

@@ -99,7 +99,7 @@ Tagging the Release
    ```bash
    $ git checkout v$MAJOR.$MINOR.x
    $ git pull upstream v$MAJOR.$MINOR.x
-   $ git checkout -b release
+   $ git checkout -b release-v$MAJOR.$MINOR.$PATCH
 
    # Bump documented gRPC versions.
    # Also update protoc version to match protobuf version in gradle/libs.versions.toml.
@@ -135,7 +135,7 @@ Tagging the Release
 
    ```bash
    $ git checkout v$MAJOR.$MINOR.x
-   $ git merge --ff-only release
+   $ git merge --ff-only release-v$MAJOR.$MINOR.$PATCH
    $ git push upstream v$MAJOR.$MINOR.x
    $ git push upstream v$MAJOR.$MINOR.$PATCH
    ```