Normalize arcade show -T input (#331)

Small change that normalizes the toolkit package name to use underscores
instead of hyphens. Python packages use underscores, not hyphens

arcade/arcade/cli/utils.py

@@ -55,7 +55,7 @@ def create_cli_catalog(
     Load toolkits from the python environment.
     """
     if toolkit:
-        toolkit = toolkit.lower()
+        toolkit = toolkit.lower().replace("-", "_")
         try:
             prefixed_toolkit = "arcade_" + toolkit
             toolkits = [Toolkit.from_package(prefixed_toolkit)]