Added GITHUB_API to CONTRIBUTING.md docs

CONTRIBUTING.md

@@ -11,13 +11,17 @@
 Create `.env` file to project root directory, and write your GitHub token to the `.env` file.
 Please select the authority of `repo` when creating token.
 
-```
+```properties
 GITHUB_TOKEN=XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+
+# if using GitHub Enterprise:
+# (this env var defaults to https://api.github.com/graphql)
+GITHUB_API=https://github.example.com/api/graphql
 ```
 
 Run local server.
 
-```
+```sh
 deno run --allow-net --allow-read --allow-env debug.ts
 ```
 
@@ -33,6 +37,6 @@ Read the [.editorconfig](./.editorconfig)
 
 If you want to contribute to my project, you should check the lint with the following command.
 
-```
+```sh
 deno lint --unstable
-```
+```