Rename some variables in docs

docs/users_guide/glasgow_exts.xml

@@ -2007,9 +2007,9 @@ upd1 t x = t { f1=x }   -- OK:   upd1 :: T a -> b -> T b
 upd2 t x = t { f2=x }   -- BAD   (f2's type mentions b, which is
                                   existentially quantified)
 
-data G a b where { G1 { f1::a, f2::c } :: G a [c] }
-upd3 g x = g { f1=x }   -- OK:   upd3 :: G a b -> c -> G c b
-upd4 g x = g { f2=x }   -- BAD (f2's type mentions c, which is not a simple
+data G a b where { G1 { g1::a, g2::c } :: G a [c] }
+upd3 g x = g { g1=x }   -- OK:   upd3 :: G a b -> c -> G c b
+upd4 g x = g { g2=x }   -- BAD (f2's type mentions c, which is not a simple
                         --      type-variable argument in G1's result type)
 </programlisting>
 </para>