Change master->main branch references in validate-schema.yml

Change master->main branch references in validate-schema.yml. This fixes Issue #326.

Author: ccoffin

.github/workflows/validate-schema.yml

@@ -2,12 +2,12 @@ name: Validate JSON Schemas
 on:
   push:
     branches:
-      - master
+      - main
   pull_request:
     types: [opened, synchronize, reopened]
   workflow_dispatch:
     branches:
-      - master
+      - main
 env:
   CVE_SCHEMA_DIR: schema
   CVE_SCHEMA_FILENAME: CVE_Record_Format.json