Update Parameter.rakudoc

In the edited example, I see no way how the ::U type capture can be different from the ::T one. So it appears that the possibility of having more than one type capture per argument is useless. If an explanation can be provided, I will gladly edit it back in, with the explanation.

doc/Type/Parameter.rakudoc

@@ -282,9 +282,9 @@ Returns a list of variable names of L<type captures|/language/signatures#Type_ca
 parameter.  Type captures define a type name within the attached code,
 which is an alias to the type gleaned from the argument during a call.
 
-    sub a(::T ::U $x) { T.say }
+    sub a(::T $x) { T.say }
     a(8);                                       # OUTPUT: «(Int)␤»
-    say &a.signature.params[0].type_captures;   # OUTPUT: «(T U)␤»
+    say &a.signature.params[0].type_captures;   # OUTPUT: «(T)␤»
     sub b($x) { $x.^name.say }
     a(8);                                       # OUTPUT: «Int␤»