bookmarks: refer to "the" active bookmark to clarify that there's only one stable

bookmarks: refer to "the" active bookmark to clarify that there's only one

This is a follow-on to 0c6cdbb697d9 that just makes a slight clarification.

--- a/mercurial/commands.py	Wed Aug 27 23:10:06 2014 +0900
+++ b/mercurial/commands.py	Sat Aug 30 15:13:02 2014 +0200
@@ -818,8 +818,8 @@
     Deleting or moving a bookmark has no effect on the associated changesets.
 
     Creating or updating to a bookmark causes it to be marked as 'active'.
-    Active bookmarks are indicated with a '*'.
-    When a commit is made, an active bookmark will advance to the new commit.
+    The active bookmark is indicated with a '*'.
+    When a commit is made, the active bookmark will advance to the new commit.
     A plain :hg:`update` will also advance an active bookmark, if possible.
     Updating away from a bookmark will cause it to be deactivated.