datetime.rst: correct timedelta description

It was confusingly claimed that `timedelta` could represent the difference between two `datetime.time` objects.

Subtraction of two `datetime.time` values is not allowed, nor can a `timedelta` be added to a `time`.  And specifically, this conflicted text in the "timedelta Objects" section, stating "A timedelta object represents [...] the difference between two dates or times."

Author: belm0

Doc/library/datetime.rst

@@ -121,7 +121,7 @@ Available Types
 .. class:: timedelta
    :noindex:
 
-   A duration expressing the difference between two :class:`date`, :class:`.time`,
+   A duration expressing the difference between two :class:`date`
    or :class:`.datetime` instances to microsecond resolution.