Use a single `=` in 'impl as where' to assign an associated constant (#4548)

This was written as `==` but is inconsistent with the rest of the
documentation for assigning associated constants.

docs/design/generics/details.md

@@ -2164,7 +2164,7 @@ specifying the values of associated facets, as in:
 ```carbon
 impl VeryLongTypeName as Add
     // `Self` here means `VeryLongTypeName`
-    where .Result == Self {
+    where .Result = Self {
   ...
 }
 ```