update id of json schema to be a valid url

sigmf-schema.json

@@ -1,5 +1,5 @@
 {
-  "$id": "https://github.com/sigmf/SigMF/spec/1.2.0/sigmf-schema",
+  "$id": "https://github.com/sigmf/SigMF/blob/v1.2.1/sigmf-schema.json",
   "$schema": "https://json-schema.org/draft/2020-12/schema",
   "title": "Schema for SigMF Meta Files",
   "description": "SigMF specifies a way to describe sets of recorded digital signal samples with metadata written in JSON. SigMF can be used to describe general information about a collection of samples, the characteristics of the system that generated the samples, features of signals themselves, and the relationship between different recordings.",