Fix typo in PoFile docstring

Author: flashcode

msgcheck.py

@@ -108,7 +108,7 @@ def __init__(self, msg, charset, fuzzy, line):
 
 class PoFile(object):
     """
-    A gettext file. It includes functions to read the file, and perform
+    A gettext file. It includes methods to read the file, and perform
     checks on the translations.
     """