add missing projection declaration

add missing projection declaration in `stac_extensions` in collection example

Author: emmanuelmathot

examples/collection.json

@@ -3,6 +3,7 @@
   "type": "Collection",
   "stac_extensions": [
     "https://stac-extensions.github.io/eo/v1.0.0/schema.json",
+    "https://stac-extensions.github.io/projection/v1.0.0/schema.json",
     "https://stac-extensions.github.io/view/v1.0.0/schema.json"
   ],
   "stac_version": "1.0.0-rc.3",