help: fix makeitemsdoc English description

mercurial/help.py

@@ -179,7 +179,7 b' def addtopichook(topic, rewriter):'
 
 def makeitemsdoc(topic, doc, marker, items, dedent=False):
     """Extract docstring from the items key to function mapping, build a
-    .single documentation block and use it to overwrite the marker in doc
+    single documentation block and use it to overwrite the marker in doc.
     """
     entries = []
     for name in sorted(items):