shorten docstring

Author: charlesdong1991

pandas/core/reshape/merge.py

@@ -165,8 +165,7 @@ def merge_ordered(left, right, on=None,
  `left` and `right` respectively. Pass a value of `None` instead
  of a string to indicate that the column name from `left` or
  `right` should be left as-is, with no suffix. At least one of the
- values must not be None. A combination of `''` and `None` is will
- raise error for columns which type is string
+ values must not be None.
  how : {'left', 'right', 'outer', 'inner'}, default 'outer'
  * left: use only keys from left frame (SQL: left outer join)
  * right: use only keys from right frame (SQL: right outer join)