Fix phrasing in workflow.md

Fix phrasing for agreement.

Author: Amanda Butler

docs/contributing/guidelines/workflow.md

@@ -53,7 +53,7 @@ Pull requests on GitHub have to meet the following requirements to keep the code
 - Avoid merging commmits. (Always rebase when possible.)
 - Comment in the pull request on every change (rebase or new commits). This helps reviewers to be up to date with changes
 - Pull requests should fix a bug, add a feature or refactor.
-   The only exception are third-party version updates (for example, Mbed TLS or Nanostack releases for Mbed OS). These updates should provide Mbed OS release notes in the pull request description, or link to an external changelog or release notes.
+   The only exceptions are third-party version updates (for example, Mbed TLS or Nanostack releases for Mbed OS). These updates should provide Mbed OS release notes in the pull request description, or link to an external changelog or release notes.
 - Smaller pull requests are easier to review and faster to integrate. Use dependencies – split your work by pull request type or functional changes. To add a third-party driver, send it in a separate pull request, and add it as a dependency to your pull request.
 
 If commits do not follow the above guidelines, we may request that you modify the commit history (often to add more details to address _what_ and _why_ rather than _how_).