[PHP] Use appDescription in composer.mustache

Created by: Yogarine

PR checklist

• [√] Read the contribution guidelines.
• [√] Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• [√] Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
• [√] Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

Use {{appDescription}} in composer.mustache instead of the {{description}} which doesn't exist.

@jebentier @dkarlovi @mandrean @jfastnacht @ackintosh @ybelenko