chore: fix file name discrepancy in the code

packages/ast/scripts/test-ast.js

@@ -1,5 +1,5 @@
 // This file is a demonstation of how ast package works. In order to try it yourself you need:
-// 1. Create a 'fixture.tx' file in the same directory as this file and write any TypeScript to it.
+// 1. Create a 'fixture.ts' file in the same directory as this file and write any TypeScript to it.
 // 2. cd into `packages/ast`.
 // 3. Run `yarn test:ast`.
 // `test-output.json` will be created containing an abstract syntax tree of the `fixture.ts` file.