Replace outdated bitesized label link (#5790)

Meta: I was looking for possible contribution opportunities, so I
noticed this in the docs. I assume that the `bite-size` label is
outdated, as there aren't actually any issues with that label. Looks
like https://github.com/beetbox/beets/labels/good%20first%20issue is
currently used.

Note: Further below there's also mention of a `first timers only` label
– that link also yields no results and even gives a message about the
label being invalid. I wasn't sure if/how that should be replaced, so I
didn't touch it. Maybe the sentence I am changing should actually be
moved down and replace the one mentioning `first timers only`?

Author: snejus

CONTRIBUTING.rst

@@ -154,7 +154,7 @@ Code Contribution Ideas
 ^^^^^^^^^^^^^^^^^^^^^^^
 
 -  We maintain a set of `issues marked as
-   “bite-sized” <https://github.com/beetbox/beets/labels/bitesize>`__.
+   “good first issue” <https://github.com/beetbox/beets/labels/good%20first%20issue>`__.
    These are issues that would serve as a good introduction to the
    codebase. Claim one and start exploring!
 -  Like testing? Our `test