workflow: fix wording for pull request

Author: 0xc0170

docs/reference/contributing/guidelines/workflow.md

@@ -70,8 +70,8 @@ Pull requests on GitHub have to meet the following requirements to keep the code
 - Because we use GitHub and explicit CLAs, special commit tags that other projects may use, such as “Reviewed-by”, or “Signed-off-by”, are redundant and should be omitted. GitHub tracks who reviewed what and when, and our stack of signed CLAs shows us who has agreed to our development contribution agreement.
 - Prefixing your commit message with a domain is acceptable, and we recommend doing so when it makes sense. However, prefixing one's domain with the name of the repo is not useful. For example, making a commit entitled "mbed-drivers: Fix doppelwidget frobulation" to the `mbed-drivers` repo is not acceptable because it is already understood that the commit applies to `mbed-drivers`. Renaming the commit to "doppelwidget: Fix frobulation" would be better, if we presume that "doppelwidget" is a meaningful domain for changes, because it communicates that the change applies to the doppelwidget area of `mbed-drivers`.
 - All new features and enhancements require documentation, tests and user guides for us to accept them. Please link each pull request to all relevant documentation and testing pull requests.
-- Avoid merging commmits (use rebase if you could).
-- Pull requests shall fix a bug, add a feature or refactor but not a mixture.
+- Avoid merging commmits (always rebase when possible).
+- Pull requests shall fix a bug or add a feature or refactor.
 
 #### Pull request categories