Help: Fix COMPILE_LANG_AND_ID genex example

Fix the `COMPILE_LANGUAGE/CXX_COMPILER_ID` variant of the example to
have the same meaning as the `COMPILE_LANG_AND_ID` variant.  The
inconsistency was introduced by commit 808b818063 (Genex: CompileLang
and CompileLangAndId now match against a list of ids, 2019-05-30,
v3.15.0-rc1~11^2~1).

Fixes: #19862

Help/manual/cmake-generator-expressions.7.rst

@@ -195,7 +195,7 @@ Variable Queries
   .. code-block:: cmake
 
     target_compile_definitions(myapp
-      PRIVATE $<$<AND:$<COMPILE_LANGUAGE:CXX>,$<CXX_COMPILER_ID:Clang>>:COMPILING_CXX_WITH_CLANG>
+      PRIVATE $<$<AND:$<COMPILE_LANGUAGE:CXX>,$<CXX_COMPILER_ID:AppleClang,Clang>>:COMPILING_CXX_WITH_CLANG>
               $<$<AND:$<COMPILE_LANGUAGE:CXX>,$<CXX_COMPILER_ID:Intel>>:COMPILING_CXX_WITH_INTEL>
               $<$<AND:$<COMPILE_LANGUAGE:C>,$<C_COMPILER_ID:Clang>>:COMPILING_C_WITH_CLANG>
     )