Editorial: Fix reference to object in interface introspection (#964)

Missed issue during the interface extension RFC.

Co-authored-by: Roman Ivantsov <[email protected]>

Author: leebyron

spec/Section 4 -- Introspection.md

@@ -328,8 +328,8 @@ Fields\:
 - `fields` must return the set of fields required by this interface.
   - Accepts the argument `includeDeprecated` which defaults to {false}. If
     {true}, deprecated fields are also returned.
-- `interfaces` must return the set of interfaces that an object implements (if
-  none, `interfaces` must return the empty set).
+- `interfaces` must return the set of interfaces that an interface implements
+  (if none, `interfaces` must return the empty set).
 - `possibleTypes` returns the list of types that implement this interface. They
   must be object types.
 - All other fields must return {null}.