Fix copy-pasted error message in pre-build.sample

The "No git remote set" error message was appropriate for the previous
block (where it was presumably copy-pasted from), but in this line we
have failed the check that determines if we have a git branch checked
out, so we should output a corresponding error.

lib/kamal/cli/templates/sample_hooks/pre-build.sample

@@ -32,7 +32,7 @@ fi
 current_branch=$(git branch --show-current)
 
 if [ -z "$current_branch" ]; then
-  echo "No git remote set, aborting..." >&2
+  echo "Not on a git branch, aborting..." >&2
   exit 1
 fi