Bumps package.xml and PHP_RRD_VERSION to 2.0.4 so the PHP 8.4/8.5 build
fix from #4 can ship; the last release (2.0.3) predates it. Also corrects
the 'librdd' typo in the config.m4 probe message.

Validated in Docker: pecl package + pecl install produce a working 2.0.4
that reports its version correctly and passes the test suite on 8.1-8.5.

Signed-off-by: Thomas Vincent <thomasvincent@gmail.com>