[css-mixins-1] Create a registration named "result", not "return" (#13775)

Resolves issue #13769.

Author: andruud

css-mixins-1/Overview.bs

@@ -604,7 +604,7 @@ with its [=function parameters=] overriding "inherited" custom properties of the
 		Add the registration to |registrations|.
 	6. If |custom function| has a [=custom function/return type=],
 		create a [=custom property registration=]
-		with the name "return"
+		with the name "result"
 		(violating the usual rules for what a registration's name can be),
 		a syntax of the [=custom function/return type=],
 		an inherit flag of "false",