add note for commands. (#407)

* add note for commands.

The all listed commands is seemingly bash commands, but the 2nd and latter is command for rails command prompt.

* fix from null to nil

Running-Mastodon/Administration-guide.md

@@ -70,6 +70,7 @@ Additionally you can toggle the "reject media" option. When enabled, media files
 ## Reactivating a previously deleted user
 
     RAILS_ENV=production bundle exec rails c
+    # in rails console.
     account = Account.find_by(username: 'username', domain: nil)
     account.suspended = false
     user = User.create!(email: 'email', password: 'password', account: account)