Whatsnew: Clarify wording

Author: encukou

Doc/whatsnew/3.8.rst

@@ -244,7 +244,7 @@ Vectorcall: a fast calling protocol for CPython
 The "vectorcall" protocol is added to the Python/C API.
 It is meant to formalize existing optimizations which were already done
 for various classes.
-Any extension type implementing callables can use this protocol.
+Any extension type implementing a callable can use this protocol.
 
 This is currently provisional,
 the aim is to make it fully public in Python 3.9.