Fix typo in Modules/GNUInstallDirs.cmake (#15176)

Commit d4fdd9c189f85d659f4294f8ec6da3e7e51215ec ("GNUInstallDirs: use the
proper default for info and man paths on OpenBSD") introduced a typo.
Merge topic 'GNUInstallDirs-typo' into next

33f03129 Fix typo in Modules/GNUInstallDirs.cmake (#15176)