[docs] Update the label name for new contributors

[JohnTitor]

Summary

The beginner label is deprecated
and the good first issue label is now preferred.

[aaron.ballman]

LGTM, thanks for catching that!

llvm/docs/Contributing.rst

 -----------
 If you are working with LLVM and run into a bug, we definitely want to know
 about it. Please let us know and follow the instructions in
 :doc:`HowToSubmitABug`  to create a bug report.
 
 Bug Fixes
 ---------
 If you are interested in contributing code to LLVM, bugs labeled with the
-`beginner`_ keyword in the `bug tracker`_ are a good way to get familiar with
+`good first issue`_ keyword in the `bug tracker`_ are a good way to get familiar with
 the code base. If you are interested in fixing a bug please comment on it to
 let people know you are working on it.
 
 Then try to reproduce and fix the bug with upstream LLVM. Start by building
 LLVM from source as described in :doc:`GettingStarted` and
 use the built binaries to reproduce the failure described in the bug. Use
 a debug build (`-DCMAKE_BUILD_TYPE=Debug`) or a build with assertions
 (`-DLLVM_ENABLE_ASSERTIONS=On`, enabled for Debug builds).

 
 `Intro to LLVM`__
   Book chapter providing a compiler hacker's introduction to LLVM.
 
   .. __: http://www.aosabook.org/en/llvm.html
 
 .. _Forum: https://discourse.llvm.org
 .. _irc.oftc.net: irc://irc.oftc.net/llvm
-.. _beginner: https://github.com/llvm/llvm-project/issues?q=is%3Aopen+is%3Aissue+label%3Abeginner
+.. _good first issue: https://github.com/llvm/llvm-project/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22
 .. _bug tracker: https://github.com/llvm/llvm-project/issues
 .. _clang-format-diff.py: https://reviews.llvm.org/source/llvm-github/browse/main/clang/tools/clang-format/clang-format-diff.py
 .. _git-clang-format: https://reviews.llvm.org/source/llvm-github/browse/main/clang/tools/clang-format/git-clang-format
 .. _LLVM's Phabricator: https://reviews.llvm.org/
 .. _LLVM's Open Projects page: https://llvm.org/OpenProjects.html#what