Add dash to package keyword

C.f. jupyterlab/jupyterlab#3841.

Author: vidartf

js/package.json

@@ -38,7 +38,7 @@
   "keywords": [
     "jupyterlab",
     "extension",
-    "jupyterlab extension",
+    "jupyterlab-extension",
     "notebook",
     "jupyter",
     "widgets",