Change "pls" to "please" in error message (pandas-dev#16947)

Author: alanbato

pandas/core/frame.py

@@ -3359,7 +3359,7 @@ def sort_index(self, axis=0, level=None, ascending=True, inplace=False,
         inplace = validate_bool_kwarg(inplace, 'inplace')
         # 10726
         if by is not None:
-            warnings.warn("by argument to sort_index is deprecated, pls use "
+            warnings.warn("by argument to sort_index is deprecated, please use "
                           ".sort_values(by=...)", FutureWarning, stacklevel=2)
             if level is not None:
                 raise ValueError("unable to simultaneously sort by and level")