[clang-tidy] Fix AST Library documentation link

[sousajo]

Summary

The link was pointing to the 'Lexer and Preprocessor Library'
instead.

[sousajo]

I dont have commit access, can someone land it for me?

Name: Jorge Pinto Sousa
Email: <[e-mail]>

clang-tools-extra/docs/clang-tidy/Contributing.rst

 - `The Clang "Basic" Library
   <https://clang.llvm.org/docs/InternalsManual.html#the-clang-basic-library>`_ for
   information about diagnostics, fix-it hints and source locations.
 - `The Lexer and Preprocessor Library
   <https://clang.llvm.org/docs/InternalsManual.html#the-lexer-and-preprocessor-library>`_
   for information about tokens, lexing (transforming characters into tokens) and the
   preprocessor.
 - `The AST Library
-  <https://clang.llvm.org/docs/InternalsManual.html#the-lexer-and-preprocessor-library>`_
+  <https://clang.llvm.org/docs/InternalsManual.html#the-ast-library>`_
   for information about how C++ source statements are represented as an abstract syntax
   tree (AST).
 
 Most checks will interact with C++ source code via the AST.  Some checks will interact
 with the preprocessor.  The input source file is lexed and preprocessed and then parsed
 into the AST.  Once the AST is fully constructed, the check is run by applying the check's
 registered AST matchers against the AST and invoking the check with the set of matched
 nodes from the AST.  Monitoring the actions of the preprocessor is detached from the