fix typo

Co-authored-by: Bryan Mishkin <[email protected]>

Author: Vincent Molinié

src/parse-result.test.ts

@@ -1521,7 +1521,7 @@ describe('ember-template-recast', function () {
     });
   });
 
-  describe('NullLitteral', function () {
+  describe('NullLiteral', function () {
     test('creating void element', function () {
       let template = `{{my-helper null}}`;