Make update-reference-docs.sh OSX compatible.

Based on
https://unix.stackexchange.com/questions/663369/invalid-command-code-using-sed-to-replace-characters-inline

Verified this works with OSX 12.5, and Ubuntu 22.04.1.

Signed-off-by: Billy Lynch <billy@chainguard.dev>

hack/update-reference-docs.sh

@@ -23,4 +23,4 @@ go run github.com/ahmetb/gen-crd-api-reference-docs \
     -api-dir "github.com/tektoncd/pipeline/pkg/apis" \
     -template-dir "./hack/reference-docs-template" \
     -out-file "./docs/pipeline-api.md"
-sed -i '1s/^/<!--\n---\ntitle: Pipeline API\nlinkTitle: Pipeline API\nweight: 1000\n---\n-->\n\n/' ./docs/pipeline-api.md
+sed -i".backup" '1s/^/<!--\n---\ntitle: Pipeline API\nlinkTitle: Pipeline API\nweight: 1000\n---\n-->\n\n/' ./docs/pipeline-api.md