Add "either"

Author: danny0838

Doc/library/zipfile.rst

@@ -595,9 +595,9 @@ ZipFile objects
 
    When scanning, *strict_descriptor* controls how entries written with an
    unsigned *data descriptor* are handled.  A data descriptor is an optional
-   record stored after an entry's data that can be signed (beginning with a
-   magic signature) or unsigned, and is mandatory for an archive written to a
-   non-seekable stream.  Unsigned descriptors have been deprecated by the
+   record stored after an entry's data that can be either signed (beginning
+   with a magic signature) or unsigned, and is mandatory for an archive written
+   to a non-seekable stream.  Unsigned descriptors have been deprecated by the
    `PKZIP Application Note`_ since version 6.3.0 (released in 2006) and are
    rarely produced by modern tools.