Revise variable names and error message in change stream examples
Revise variable names and error message in change stream examples

jmikola/mongo-php-library,mongodb/mongo-php-library,jmikola/mongo-php-library-prototype,mongodb/mongo-php-library,jmikola/mongo-php-library,jmikola/mongo-php-library,10gen-labs/mongo-php-library-prototype,mongodb/mongo-php-library,jmikola/mongo-php-library,mongodb/mongo-php-library