Merge bitcoin/bitcoin#31327: doc: Correct PR Review Club frequency from weekly to monthly

637f437 doc: remove PR Review Club frequency (Gabriele Bocchi)

Pull request description:

  The PR Review Club is mentioned as weekly in the CONTRIBUTING.md file, but it is held monthly as per the official [Bitcoin Core PR Review Club website](https://bitcoincore.reviews/). This PR updates the documentation to just remove the frequency.

ACKs for top commit:
  fanquake:
    ACK 637f437

Tree-SHA512: 27bf8a0e32edd8bedb5301ceb3c744ff4629403292a7ad00b633921f36278443ae297cd53708a533b1d6e6eab863b831e11247b95277b94cce28e3d5ddb7d9b9

Author: fanquake

CONTRIBUTING.md

@@ -36,8 +36,7 @@ list or changes that are
 Some of them might no longer be applicable. So if you are interested, but
 unsure, you might want to leave a comment on the issue first.
 
-You may also participate in the weekly
-[Bitcoin Core PR Review Club](https://bitcoincore.reviews/) meeting.
+You may also participate in the [Bitcoin Core PR Review Club](https://bitcoincore.reviews/).
 
 ### Good First Issue Label