Fix return type of IPv4Address.ipv6_mapped in ipaddress docs (GH-157887)

(cherry picked from commit 34cff00)

Co-authored-by: Prafyl <86287866+Prafyl@users.noreply.github.com>

Author: Prafyl

Doc/library/ipaddress.rst

@@ -255,7 +255,7 @@ write code that handles both IP versions correctly.  Address objects are
 
    .. attribute:: ipv6_mapped
 
-      :class:`IPv4Address` object representing the IPv4-mapped IPv6 address. See :RFC:`4291`.
+      :class:`IPv6Address` object representing the IPv4-mapped IPv6 address. See :RFC:`4291`.
 
       .. versionadded:: 3.13