FindGettext: Fix name of GETTEXT_PROCESS_POT_FILE in docs (#15162)

Rename GETTEXT_PROCESS_POT to GETTEXT_PROCESS_POT_FILE.

Patch-by: [email protected]

Modules/FindGettext.cmake

@@ -26,7 +26,7 @@
 #     ALL option is used, the translations will also be created when
 #     building the default target.
 #
-# GETTEXT_PROCESS_POT( <potfile> [ALL] [INSTALL_DESTINATION <destdir>]
+# GETTEXT_PROCESS_POT_FILE( <potfile> [ALL] [INSTALL_DESTINATION <destdir>]
 # LANGUAGES <lang1> <lang2> ...  )
 #
 # ::