GridHelper: Fixed typo.

src/helpers/GridHelper.js

@@ -69,9 +69,9 @@ GridHelper.prototype = Object.assign( Object.create( LineSegments.prototype ), {
 
 	clone() {
 
-		var paramters = this.parameters;
+		var parameters = this.parameters;
 
-		return new this.constructor( paramters.size, paramters.divisions, paramters.color1, paramters.color2 );
+		return new this.constructor( parameters.size, parameters.divisions, parameters.color1, parameters.color2 );
 
 	}