Fix typo in runtime type exception (#1447)

* Fix typo in runtime type exception

* fix test

Author: juliaqiuxy

src/execution/__tests__/abstract-test.js

@@ -413,7 +413,7 @@ describe('Execute: Handles execution of abstract types', () => {
             'Abstract type FooInterface must resolve to an Object type at ' +
             'runtime for field Query.foo with value "dummy", received "[]". ' +
             'Either the FooInterface type should provide a "resolveType" ' +
-            'function or each possible types should provide an "isTypeOf" function.',
+            'function or each possible type should provide an "isTypeOf" function.',
           locations: [{ line: 1, column: 3 }],
           path: ['foo'],
         },

src/execution/execute.js

@@ -1048,7 +1048,7 @@ function ensureValidRuntimeType(
         `runtime for field ${info.parentType.name}.${info.fieldName} with ` +
         `value ${inspect(result)}, received "${inspect(runtimeType)}". ` +
         `Either the ${returnType.name} type should provide a "resolveType" ` +
-        'function or each possible types should provide an ' +
+        'function or each possible type should provide an ' +
         '"isTypeOf" function.',
       fieldNodes,
     );