README: fix path for bitbucket-green-merge-button.png

GitHub can't handle paths with period-slash prefix "./" in relative
paths to images in Markdown syntax.  Remove unnecessary prefix in the
path to "images/bitbucket-green-merge-button.png" in README.md.

README.md

@@ -123,7 +123,7 @@ This userstyle makes active merge buttons green in Bitbucket pull requests.
 Inspired by [code review tool "Critique"](https://engineercodex.substack.com/p/how-google-takes-the-pain-out-of).
 This style supports both Bitbucket Server and Bitbucket Cloud.
 
-![Before and After screenshots of "Bitbucket: green merge button"](./images/bitbucket-green-merge-button.png)
+![Before and After screenshots of "Bitbucket: green merge button"](images/bitbucket-green-merge-button.png)
 
 ### Bitbucket: speed up CI builds
 [![UserStyles.world link][badge_userstyles_world]][usw_bitbucket_fast_and_furious]