avocado/plugins/run.py: simple typo (white space missing) fix

The run plugin description is missing a white space.  Let's fix that.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

avocado/plugins/run.py

@@ -36,7 +36,7 @@ class Run(CLICmd):
     """
 
     name = 'run'
-    description = ("Runs one or more tests (native test, test alias, binary"
+    description = ("Runs one or more tests (native test, test alias, binary "
                    "or script)")
 
     def configure(self, parser):