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 808b8180 (Genex: CompileLang
and CompileLangAndId now match against a list of ids, 2019-05-30,
v3.15.0-rc1~11^2~1).

Fixes: #19862
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  External
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