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.

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