glossary: fixed typo in "Directory, working" description

The term "Directory, working" referenced a non-existant term named "Parents,
working directory". This patch fixes the typo.

mercurial/help/glossary.txt

@@ -185,7 +185,7 b' Directory, working'
     Mercurial, that will be recorded in the next commit. The working
     directory initially corresponds to the snapshot at an existing
     changeset, known as the parent of the working directory. See
-    'Parents, working directory'. The state may be modified by changes
+    'Parent, working directory'. The state may be modified by changes
     to the files introduced manually or by a merge. The repository
     metadata exists in the .hg directory inside the working directory.