Fix "heroku-sys/php" name in platform repo example
Fix "heroku-sys/php" name in platform repo example

Platform packages must require others with their "`heroku-sys`" prefix; thanks for bringing this up in https://github.com/heroku/heroku-buildpack-php/issues/417#issuecomment-780084876, @boboldehampsink
emeth-/heroku-buildpack-php,heroku/heroku-buildpack-php,heroku/heroku-buildpack-php,emeth-/heroku-buildpack-php,emeth-/heroku-buildpack-php,emeth-/heroku-buildpack-php,heroku/heroku-buildpack-php,heroku/heroku-buildpack-php