Rename file to conform to psr-4 autoloading standard

[knox2]

This pull request renames the RunTimeException.php file to RuntimeException.php to conform to the psr-4 php standard.

Currently composer gives the below deprecated notice:

Deprecation Notice: Class CurrencyConverter\Exception\RuntimeException located in ./vendor/ujjwal/currency-converter/src/Exception/RunTimeException.php does not comply with psr-4 autoloading standard. It will not autoload anymore in Composer v2.0. in phar:///usr/local/bin/composer/src/Composer/Autoload/ClassMapGenerator.php:201

This fixes the above issue

[ojhaujjwal]

Thanks for the PR @knox2. Do you mind removing the existing RunTimeException.php too?

[ojhaujjwal]

I removed the file myself as it was a minor change.
I also published https://github.com/ojhaujjwal/currency-converter-php/releases/tag/2.5.1.
Thanks for your contribution. :)