Simplify wording

Author: spawnia

src/type/directives.ts

@@ -253,7 +253,7 @@ export const GraphQLSpecifiedByDirective: GraphQLDirective =
 export const GraphQLOneOfDirective: GraphQLDirective = new GraphQLDirective({
   name: 'oneOf',
   description:
-    'Indicates that exactly one field must be supplied and that field must not be `null`.',
+    'Indicates that exactly one field must be supplied and not `null`.',
   locations: [DirectiveLocation.INPUT_OBJECT],
   args: {},
 });

src/utilities/__tests__/printSchema-test.ts

@@ -796,7 +796,7 @@ describe('Type System Printer', () => {
       ) on SCALAR
 
       """
-      Indicates that exactly one field must be supplied and that field must not be \`null\`.
+      Indicates that exactly one field must be supplied and not \`null\`.
       """
       directive @oneOf on INPUT_OBJECT