Match singular/plural

doc/Language/classtut.pod6

@@ -942,7 +942,7 @@ which it can find out via introspection.
 
 =head2 X<Overriding default gist method|Reference,Overriding default gist method>
 
-Some classes might need its own version of C<gist>, which overrides the terse
+Some classes might need their own version of C<gist>, which overrides the terse
 way it is printed when called to provide a default representation of the class.
 For instance, L<exceptions|/language/exceptions#Uncaught_exceptions> might want
 to write just the C<payload> and not the full object so that it is clearer what