Typo in last docstring.

Author: nedbat

coverage/control.py

@@ -78,7 +78,7 @@ def erase(self):
     def exclude(self, regex):
         """Exclude source lines from execution consideration.
         
-        `regex` is a regular expression.  Lines matching this expressions are
+        `regex` is a regular expression.  Lines matching this expression are
         not considered executable when reporting code coverage.  A list of
         regexes is maintained; this function adds a new regex to the list.
         Matching any of the regexes excludes a source line.