DOC: lua: fix filters.txt file location

At the beginning of the filter class section, we encourage the user to
check out filters.txt file to get to know how the filters API works
within haproxy.

However the file location is incorrect. The proper directory to look for
the file is: doc/internals/api.

It should be backported up to 2.5.

(cherry picked from commit 84f7525c5b3831a76f17dacc8891aad27e1f4de5)
Signed-off-by: Amaury Denoyelle <adenoyelle@haproxy.com>
(cherry picked from commit 529ef652d8d8351af0b25a4e44310bbe459148c4)
Signed-off-by: Amaury Denoyelle <adenoyelle@haproxy.com>
1 file changed