Update $id to use rolling release

Per ossf#132 (comment), OSV
uses a rolling release that is fully backwards compatible, preventing the use
of SEMVER for versioning.

Updating $id to reflect this, as there will not be a v2 pushed to main, and all
future versions are defined as backwards compatible with older versions.

Signed-off-by: Josh Buker <crypto@joshbuker.com>

validation/schema.json

@@ -1,6 +1,6 @@
 {
   "$schema": "https://json-schema.org/draft/2020-12/schema",
-  "$id": "https://raw.githubusercontent.com/ossf/osv-schema/v1.4.1/validation/schema.json",
+  "$id": "https://raw.githubusercontent.com/ossf/osv-schema/main/validation/schema.json",
   "title": "Open Source Vulnerability",
   "description": "A schema for describing a vulnerability in an open source package.",
   "type": "object",