docs: fix Note format in GitHub markdown

Author: kenjis

README.md

@@ -143,7 +143,8 @@ predefined rulesets. Rector can be highly opinionated based on its configuration
 so be sure to read the documentation and figure out the best fit for you. This workflow performs
 a "dry run" to check for any changes that Rector would have made and fail if there are matches.
 
-> Note: Rector updates rules all the time, so you may want to lock your repo to
+> **Note**
+> Rector updates rules all the time, so you may want to lock your repo to
 > the latest known working version of Rector to prevent unexpected failures.
 > Using pinned version in `composer.json` and update it with dependabot is the
 > best practice.