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

Commit d4fdd9c189f85d659f4294f8ec6da3e7e51215ec ("GNUInstallDirs: use the
proper default for info and man paths on OpenBSD") introduced a typo.

Modules/GNUInstallDirs.cmake

@@ -198,7 +198,7 @@ if(CMAKE_SYSTEM_NAME STREQUAL "OpenBSD")
     set(CMAKE_INSTALL_INFODIR "info")
   endif()
 
-  if(NOT CMAKE_INSTALL_MANDDIR)
+  if(NOT CMAKE_INSTALL_MANDIR)
     set(CMAKE_INSTALL_MANDIR "" CACHE PATH "man documentation (man)")
     set(CMAKE_INSTALL_MANDIR "man")
   endif()
@@ -208,7 +208,7 @@ else()
     set(CMAKE_INSTALL_INFODIR "${CMAKE_INSTALL_DATAROOTDIR}/info")
   endif()
 
-  if(NOT CMAKE_INSTALL_MANDDIR)
+  if(NOT CMAKE_INSTALL_MANDIR)
     set(CMAKE_INSTALL_MANDIR "" CACHE PATH "man documentation (DATAROOTDIR/man)")
     set(CMAKE_INSTALL_MANDIR "${CMAKE_INSTALL_DATAROOTDIR}/man")
   endif()