Makefile: update schema update instructions
We don't carry vendoring for gangplank anymore.
@@ -89,7 +89,7 @@ schema:
# Edit src/v1.json
# $ cp src/v1.json schema/
# $ make schema
-# $ for d in mantle gangplank; do (cd $d && go mod vendor); done
+# $ (cd mantle && go mod vendor)
.PHONY: schema-check
schema-check: DIGEST = $(shell sha256sum src/v1.json | awk '{print $$1}')
schema-check:
