DOC: Fix typo in merging.rst

Author: ananiavito

doc/source/user_guide/merging.rst

@@ -979,7 +979,7 @@ nearest key rather than equal keys. For each row in the ``left`` :class:`DataFra
 the last row in the ``right`` :class:`DataFrame` are selected where the ``on`` key is less
 than the left's key. Both :class:`DataFrame` must be sorted by the key.
 
-Optionally an :func:`merge_asof` can perform a group-wise merge by matching the
+Optionally :func:`merge_asof` can perform a group-wise merge by matching the
 ``by`` key in addition to the nearest match on the ``on`` key.
 
 .. ipython:: python