Fix comment in mod_lua and docs.

s/addoutputfilter/add_output_filter/
See PR 62359
(r1837404 on trunk)

Note: .c file is patch without any vote because it is only a comment in order to be consistent with the doc and the code, and mod_lua is marked as experimental.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1837405 13f79535-47bb-0310-9956-ffa450edef68