Fix error message typos.

Author: dlongley

lib/context.js

@@ -494,7 +494,7 @@ api.createTermDefinition = (
     let type = value['@type'];
     if(!_isString(type)) {
       throw new JsonLdError(
-        'Invalid JSON-LD syntax; an @context @type values must be a string.',
+        'Invalid JSON-LD syntax; an @context @type value must be a string.',
         'jsonld.SyntaxError',
         {code: 'invalid type mapping', context: localCtx});
     }
@@ -513,7 +513,7 @@ api.createTermDefinition = (
       }
       if(type.indexOf('_:') === 0) {
         throw new JsonLdError(
-          'Invalid JSON-LD syntax; an @context @type values must be an IRI, ' +
+          'Invalid JSON-LD syntax; an @context @type value must be an IRI, ' +
           'not a blank node identifier.',
           'jsonld.SyntaxError',
           {code: 'invalid type mapping', context: localCtx});