rename _multi bar_ subcommand to `multi-bar`

this is for consistency, to use the _command case_ everywhere, not only
in arguments and options.

README.md

@@ -77,7 +77,7 @@ let data = {
 }
 ```
 ```bash
-gplt multi_bar ...[
+gplt multi-bar ...[
     --title "TITLE"
     --label "LABEL"
     --fullscreen

src/main.py

@@ -34,7 +34,7 @@ if __name__ == "__main__":
     plot_parser.add_argument("--dpi", type=int, default=500, help="the DPI of the save image")
 
     multi_bar_parser = subparsers.add_parser(
-        "multi_bar",
+        "multi-bar",
         help="Multi-bar plot.",
         formatter_class=argparse.RawTextHelpFormatter,
     )
@@ -64,7 +64,7 @@ if __name__ == "__main__":
             save_aspect_ratio=args.aspect_ratio,
             save_dpi=args.dpi,
         )
-    elif args.subcommand == "multi_bar":
+    elif args.subcommand == "multi-bar":
         groups, measurements = multi_bar.extract(json.loads(args.data))
 
         plot_layout = "constrained" if args.fullscreen else None