Improved docstring.

slapos/libnetworkcachetests.py

@@ -122,7 +122,7 @@ class TestNetworkcacheClient(LibNetworkCacheMixin):
   def test_signature_creation_without_private_key_file(self):
     """
       Without the private key file, it is not possible to create the
-      signature so it must 
+      signature so it must return an empty string.
     """
     self.assertEquals('', self.nc._getSignatureString())