Update missing.py

Author: mroeschke

pandas/core/missing.py

@@ -171,7 +171,7 @@ def _interp_limit(invalid, fw_limit, bw_limit):
 
     if limit is not None:
         if is_integer(limit) and limit < 1:
-            raise ValueError('Limit must be non-negative')
+            raise ValueError('Limit must be non-negative.')
         if limit_direction == 'forward':
             violate_limit = sorted(start_nans | set(_interp_limit(invalid,
                                                                   limit, 0)))