DOC: typo: ACL subdir match

ACL subdir match is "-m dir"

backport: 1.5 and above
(cherry picked from commit 33db6004425ef290f12f305907cf8c961b8f8501)
(cherry picked from commit 0e6871bd28819cd35f5a05617806f19561863f53)
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 5003323..fad7d59 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -9795,7 +9795,7 @@
   - suffix match    (-m end) : the patterns are compared with the end of the
     extracted string, and the ACL matches if any of them matches.
 
-  - subdir match    (-m sub) : the patterns are looked up inside the extracted
+  - subdir match    (-m dir) : the patterns are looked up inside the extracted
     string, delimited with slashes ("/"), and the ACL matches if any of them
     matches.