Fixed abstract-php-extension.rb link in README
Fixed abstract-php-extension.rb link in README

Closes #1507.

Signed-off-by: Alan Ivey <ee249fd9166eccef852f50ca0cbb6ff8665afe99@gmail.com>

philcook/homebrew-php,TomK/homebrew-php,mkalkbrenner/homebrew-php,dguyon/homebrew-php,ablyler/homebrew-php