Fix headless module path

Fixes #5441

headless/package.json

@@ -3,7 +3,7 @@
  "description": "A headless terminal component that runs in Node.js",
  "version": "5.5.0",
  "main": "lib-headless/xterm-headless.js",
- "module": "lib/xterm.mjs",
+ "module": "lib-headless/xterm-headless.mjs",
  "types": "typings/xterm-headless.d.ts",
  "repository": "https://github.com/xtermjs/xterm.js",
  "license": "MIT",
@@ -22,4 +22,4 @@
   "webgl",
   "xterm"
  ]
-}
+}