rename github-token input to github_token (fix #4)

because github-token is mapped to invalid environment variable name
`INPUT_GITHUB-TOKEN`. The variable cannot be referred via bash because
`-` has special meaning in `${...}` expansion.
This change is not breaking change because previous `github-token` input
was completely wrong.

action/action.yml

@@ -8,7 +8,7 @@ branding:
   color: 'blue'
 
 inputs:
-  github-token:
+  github_token:
     description: GitHub API token for pushing the commit to update changelog. Please pass secrets.GITHUB_TOKEN
     required: true
   file: