MAINTAINERS: update PPP OVER L2TP file patterns

Commit 21b4aaa1 ("l2tp: Relocate pppol2tp
driver to new net/l2tp directory") moved the file.

Signed-off-by: Joe Perches <joe@perches.com>
Cc: James Chapman <jchapman@katalix.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>