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

Rename GETTEXT_PROCESS_POT to GETTEXT_PROCESS_POT_FILE.

Patch-by: maahnat@gmail.com
Merge topic 'FindGettext-doc-update' into next

3919f254 FindGettext: Fix name of GETTEXT_PROCESS_POT_FILE in docs (#15162)