Add whatsnew and small documentation fix

Author: mroeschke

doc/source/whatsnew/v0.21.0.txt

@@ -999,6 +999,7 @@ Performance Improvements
 - :attr:`Timestamp.microsecond` no longer re-computes on attribute access (:issue:`17331`)
 - Improved performance of the :class:`CategoricalIndex` for data that is already categorical dtype (:issue:`17513`)
 - Improved performance of :meth:`RangeIndex.min` and :meth:`RangeIndex.max` by using ``RangeIndex`` properties to perform the computations (:issue:`17607`)
+- Added a keyword argument, `cache`, to :func:`to_datetime` that improved the performance of converting duplicate datetime arguments (:issue: `11665`)
 
 .. _whatsnew_0210.docs:
 

pandas/core/tools/datetimes.py

@@ -114,7 +114,7 @@ def to_datetime(arg, errors='raise', dayfirst=False, yearfirst=False,
         .. versionadded: 0.20.0
     cache : boolean, default False
         If True, use a cache of unique, converted dates to apply the datetime
-        conversion. Produces signficant speed-ups when parsing duplicate date.
+        conversion. Produces signficant speed-ups when parsing duplicate dates.
 
         .. versionadded: 0.21.0
     Returns