fix extra word/capitalization with Within

src/validation/__tests__/ValuesOfCorrectTypeRule-test.ts

@@ -1105,7 +1105,7 @@ describe('Validate: Values of correct type', () => {
       `).toDeepEqual([
         {
           message:
-            'Variable "$string" must be non-nullable to be used for Within OneOf Input Object type "OneOfInput".',
+            'Variable "$string" must be non-nullable to be used within OneOf Input Object type "OneOfInput".',
           locations: [{ line: 4, column: 37 }],
         },
       ]);

src/validation/rules/ValuesOfCorrectTypeRule.ts

@@ -224,7 +224,7 @@ function validateOneOfInputObject(
     if (isNullableVariable) {
       context.reportError(
         new GraphQLError(
-          `Variable "$${variableName}" must be non-nullable to be used for Within OneOf Input Object type "${type}".`,
+          `Variable "$${variableName}" must be non-nullable to be used within OneOf Input Object type "${type}".`,
           { nodes: [node] },
         ),
       );