Add comma to workflow.md

Add comma for clarity.

Author: Amanda Butler

docs/reference/contributing/guidelines/workflow.md

@@ -90,7 +90,7 @@ Release: feature
 
 Updating target implementation (adding a new target or updating already supported target) is a change for a patch release.
 
-A test report for the new target must be part of the pull request. The new target must pass all Mbed OS functional and system validation tests (using `mbed test` command) for the current Mbed OS major release including all Mbed OS supported toolchains.
+A test report for the new target must be part of the pull request. The new target must pass all Mbed OS functional and system validation tests (using `mbed test` command) for the current Mbed OS major release, including all Mbed OS supported toolchains.
 
 Release: patch