PEP 448: Fix misleading passage in the abstract (#3573)

The passage in the PEP misleads its readers into believing, that
unpacking in comprehensions is supported. This is not the case, as
presented by the abstract itself:

> This PEP does not include unpacking operators inside list, set and
> dictionary comprehensions although this has not been ruled out for
> future proposals.

(Historically, the PEP originally did propose this, but that part of
the PEP was rejected, and the abstract wasn't completely updated.)

peps/pep-0448.rst

@@ -16,9 +16,7 @@ Abstract
 This PEP proposes extended usages of the ``*`` iterable unpacking
 operator and ``**`` dictionary unpacking operators
 to allow unpacking in more positions, an arbitrary number of
-times, and in additional circumstances.  Specifically,
-in function calls, in comprehensions and generator expressions, and
-in displays.
+times, and in function calls and displays.
 
 Function calls are proposed to support an arbitrary number of
 unpackings rather than just one::