Use uri over deprecated image

The use of `image` in `builder.toml` is
deprecated.  Use `uri` as the documented
replacement.

Signed-off-by: Aidan Delaney <[email protected]>

Author: AidanDelaney

content/docs/operator-guide/create-a-builder.md

@@ -30,7 +30,7 @@ uri = "samples/buildpacks/hello-processes"
 [[buildpacks]]
 # Packaged buildpacks to include in builder;
 # the "hello-universe" package contains the "hello-world" and "hello-moon" buildpacks
-image = "cnbs/sample-package:hello-universe"
+uri = "cnbs/sample-package:hello-universe"
 
 # Order used for detection
 [[order]]