Correct grammar in error message

src/math/Vector3.js

@@ -244,7 +244,7 @@ THREE.Vector3.prototype = {
 
 			if ( euler instanceof THREE.Euler === false ) {
 
-				console.error( 'THREE.Vector3: .applyEuler() now expects a Euler rotation rather than a Vector3 and order.' );
+				console.error( 'THREE.Vector3: .applyEuler() now expects an Euler rotation rather than a Vector3 and order.' );
 
 			}