[clang-tidy][NFC] Update path of main translation unit

[j-carl]

Summary

The forcing of the linker for a new module was moved from file
clang-tidy/tools/ClangTidyModule.cpp to
clang-tidy/ClangTidyForceLinker.h.

[PiotrZSL]

LGTM, probably LLVM_ATTRIBUTE_UNUSED need to be added to example before committing.

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

 .. code-block:: c++
 
   // This anchor is used to force the linker to link in the generated object file
   // and thus register the MyModule.
   volatile int MyModuleAnchorSource = 0;
 
 And this to the main translation unit of the :program:`clang-tidy` binary (or
 the binary you link the ``clang-tidy`` library in)
-``clang-tidy/tool/ClangTidyMain.cpp``:
+``clang-tidy/ClangTidyForceLinker.h``:
 
 .. code-block:: c++
 
   // This anchor is used to force the linker to link the MyModule.
   extern volatile int MyModuleAnchorSource;
   static int MyModuleAnchorDestination = MyModuleAnchorSource;