Merge pull request #833 from APIs-guru/fixErrorMsg

Fix grammar inside error message.

Author: leebyron

src/type/__tests__/validation-test.js

@@ -1239,13 +1239,13 @@ describe('Type System: Enum types must be well defined', () => {
     ).to.throw('Names must match /^[_a-zA-Z][_a-zA-Z0-9]*$/ but "#value" does not.');
 
     expect(() => enumValue('true')
-    ).to.throw('Name "true" is can not be used for Enum value.');
+    ).to.throw('Name "true" can not be used as an Enum value.');
 
     expect(() => enumValue('false')
-    ).to.throw('Name "false" is can not be used for Enum value.');
+    ).to.throw('Name "false" can not be used as an Enum value.');
 
     expect(() => enumValue('null')
-    ).to.throw('Name "null" is can not be used for Enum value.');
+    ).to.throw('Name "null" can not be used as an Enum value.');
   });
 
   it('does not allow isDeprecated without deprecationReason on enum', () => {

src/type/definition.js

@@ -957,7 +957,7 @@ function defineEnumValues(
     assertValidName(valueName);
     invariant(
       [ 'true', 'false', 'null' ].indexOf(valueName) === -1,
-      `Name "${valueName}" is can not be used for Enum value.`
+      `Name "${valueName}" can not be used as an Enum value.`
     );
 
     const value = valueMap[valueName];