[clang] Fix Typo in AST Matcher Reference

In [[ https://clang.llvm.org/docs/LibASTMatchersReference.html | AST Matcher Reference]], the example of matcher `hasDeclContext` contained a typo.

`cxxRcordDecl` was changed to `cxxRecordDecl`.

Differential Revision: https://reviews.llvm.org/D102836