MacOS and Linux compatible command

Signed-off-by: Florent Poinsard <florent.poinsard@outlook.fr>

Makefile

@@ -33,7 +33,7 @@ generate:
 	go run sigs.k8s.io/controller-tools/cmd/controller-gen object crd:maxDescLen=0 paths="./pkg/apis/planetscale/v2" output:crd:artifacts:config=./deploy/crds
 	go run github.com/ahmetb/gen-crd-api-reference-docs -api-dir planetscale.dev/vitess-operator/pkg/apis/planetscale/v2 -config ./docs/api/config.json -template-dir ./docs/api/template -out-file ./docs/api/index.html
 	@echo 'Copying ./docs/api/index.html to ./docs/api.md'
-	@cp ./docs/api/index.html ./docs/api.md && sed -i "" -e "1,2d" ./docs/api.md
+	@cp ./docs/api/index.html ./docs/api.md && ex -sc '1,2d|x' ./docs/api.md
 	@echo 'Written to ./docs/api.md'
 
 generate-and-diff: generate