Fix .env examples for PRIVATE_KEY -> GITHUB_PRIVATE_KEY variables (#4305)

This fixes naming for a couple of variables in the `.env` examples in this document.  Currently there are examples which suggest naming the private key variable `PRIVATE_KEY`, when really it should be `GITHUB_PRIVATE_KEY`.  When following along in the example and attempting to run the server, using the incorrect name in the `.env` file leads to an error.

Co-authored-by: hubwriter <hubwriter@github.com>

Author: schlagelk

content/developers/apps/setting-up-your-development-environment-to-create-a-github-app.md

@@ -151,7 +151,7 @@ You need to add these variables to the `.env` file:
 Here is an example `.env` file:
 
 ```
-PRIVATE_KEY="-----BEGIN RSA PRIVATE KEY-----
+GITHUB_PRIVATE_KEY="-----BEGIN RSA PRIVATE KEY-----
 ...
 HkVN9...
 ...
@@ -402,7 +402,7 @@ Here are a few common problems and some suggested solutions. If you run into any
     **A:** You probably haven't set up your private key environment variable quite right. Your `GITHUB_PRIVATE_KEY` variable should look like this:
 
     ```
-    PRIVATE_KEY="-----BEGIN RSA PRIVATE KEY-----
+    GITHUB_PRIVATE_KEY="-----BEGIN RSA PRIVATE KEY-----
     ...
     HkVN9...
     ...