Environment variables GITHUB_APP_ID & GITHUB_APP_PRIVATE_KEY

## Issue
I am doing the [building your first github app guide](https://developer.github.com/apps/building-your-first-github-app/), and in documentation at section [runtime environment](https://developer.github.com/apps/building-your-first-github-app/#prepare-the-runtime-environment) the variables have different naming than in code.

### Vars on Guide

export GITHUB_APP_IDENTIFIER=12345
export GITHUB_PRIVATE_KEY= ...
### Vars on Code
```
begin
GITHUB_APP_ID = ENV.fetch("GITHUB_APP_ID")
GITHUB_PRIVATE_KEY = ENV.fetch("GITHUB_APP_PRIVATE_KEY")
rescue KeyError
$stderr.puts "To run this script, please set the following environment variables:"
$stderr.puts "- GITHUB_APP_ID: GitHub App ID"
$stderr.puts "- GITHUB_APP_PRIVATE_KEY: GitHub App Private Key"
exit 1
end
```
## Solution

Decide if the docs have to be changed or the code, if the code has to be changed I will be pleased to make a pull request to fix this.

Start by comparing the guide's “Prepare the runtime environment” section with the Ruby code snippet and its environment-variable error messages. Decide which names are authoritative, then make the guide and code consistent and verify that the documented variables match those required when the sample runs.

索引モデルが issue の本文から書いたものです。