docs: update syntax for labs channel in examples

The dockerfile:1-labs frontend is now updated to 1.5-labs, which
includes this feature

Signed-off-by: Sebastiaan van Stijn <[email protected]>

Author: thaJeztah

frontend/dockerfile/docs/reference.md

@@ -1323,17 +1323,19 @@ ADD [--keep-git-dir=<boolean>] <git ref> <dir>
 ```
 
 ```dockerfile
-# syntax=docker/dockerfile-upstream:master-labs
+# syntax=docker/dockerfile:1-labs
 FROM alpine
 ADD --keep-git-dir=true https://github.com/moby/buildkit.git#v0.10.1 /buildkit
 ```
 
 The `--keep-git-dir=true` flag adds the `.git` directory. This flag defaults to false.
 
 ### Adding a private git repository
+
 To add a private repo via SSH, create a Dockerfile with the following form:
+
 ```dockerfile
-# syntax = docker/dockerfile-upstream:master-labs
+# syntax=docker/dockerfile:1-labs
 FROM alpine
 ADD [email protected]:foo/bar.git /bar
 ```