Slight correct grammar (GH-28860) 

(cherry picked from commit 1b11582f0e00f08833da6adc44030a8b1961989a)

Co-authored-by: nobodyatandnothing <91722596+nobodyatandnothing@users.noreply.github.com>
diff --git a/Doc/library/ipaddress.rst b/Doc/library/ipaddress.rst
index 74d922d..9c2dff5 100644
--- a/Doc/library/ipaddress.rst
+++ b/Doc/library/ipaddress.rst
@@ -682,7 +682,7 @@
 
       Note that currently expanded netmasks are not supported.  That means
       ``2001:db00::0/24`` is a valid argument while ``2001:db00::0/ffff:ff00::``
-      not.
+      is not.
 
    2. An integer that fits into 128 bits.  This is equivalent to a
       single-address network, with the network address being *address* and