[clang-tidy] Fix AST Library documentation link

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

Reviewed By: PiotrZSL

Differential Revision: https://reviews.llvm.org/D147874

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

@@ -278,7 +278,7 @@ are:
   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).