main: add example --progress values in the help string

This commit adds the two most useful progress types (as of today)
to the help string of `--progress`: plain and term.

This should make the usage a bit more discoverable.

bib/cmd/bootc-image-builder/main.go

@@ -691,7 +691,7 @@ func buildCobraCmdline() (*cobra.Command, error) {
 	buildCmd.Flags().String("output", ".", "artifact output directory")
 	buildCmd.Flags().String("store", "/store", "osbuild store for intermediate pipeline trees")
 	//TODO: add json progress for higher level tools like "podman bootc"
-	buildCmd.Flags().String("progress", "", "type of progress bar to use")
+	buildCmd.Flags().String("progress", "", "type of progress bar to use (e.g. plain,term)")
 	// flag rules
 	for _, dname := range []string{"output", "store", "rpmmd"} {
 		if err := buildCmd.MarkFlagDirname(dname); err != nil {