Update URL to codimd's own URL

Since we have an own URL we should use it in here, since CodiMD and
HackMD are really drifting away from each other.

Signed-off-by: Sheogorath <sheogorath@shivering-isles.com>

package.json

@@ -142,7 +142,7 @@
     "Markdown",
     "Notes"
   ],
-  "homepage": "https://hackmd.io",
+  "homepage": "https://codimd.org",
   "maintainers": [
     {
       "name": "Max Wu",