syntax-check: avoid fsf_postal false positive on maint.mk

* maint.mk (sc_fsf_postal): Adjust ‘prohibit’ regex to not match itself.

maint.mk

diff --git a/maint.mk b/maint.mk
index ad60c8f..29318a7 100644
--- a/maint.mk
+++ b/maint.mk
@@ -1065,7 +1065,7 @@
 # Look out for FSF postal addresses -- use URLs instead:
 # https://www.gnu.org/prep/maintain/html_node/License-Notices-for-Code.html
 sc_fsf_postal:
-	@prohibit='(Mass Ave|Massachusetts Ave|Temple Pl|Franklin St|Milk St)' \
+	@prohibit='([M]ass Ave|[M]assachusetts Ave|[T]emple Pl|[F]ranklin St|[M]ilk St)' \
 	halt='use license URLs instead of FSF postal address' \
 	 $(_sc_search_regexp)