Add indefinite article

Co-authored-by: Victor Stinner <vstinner@python.org>

Author: hugovk

Doc/library/plistlib.rst

@@ -18,7 +18,7 @@ and XML plist files.
 
 The property list (``.plist``) file format is a simple serialization supporting
 basic object types, like dictionaries, lists, numbers and strings.  Usually the
-top level object is a dictionary or frozen dictionary.
+top level object is a dictionary or a frozen dictionary.
 
 To write out and to parse a plist file, use the :func:`dump` and
 :func:`load` functions.

Lib/plistlib.py

@@ -2,7 +2,7 @@
 
 The property list (.plist) file format is a simple XML pickle supporting
 basic object types, like dictionaries, lists, numbers and strings.
-Usually the top level object is a dictionary or frozen dictionary.
+Usually the top level object is a dictionary or a frozen dictionary.
 
 To write out a plist file, use the dump(value, file)
 function. 'value' is the top level object, 'file' is