fixes 404 errors with JS/CSS assets urls

So, the plugin slug was set to lowercase `backupwordpress`, but the folder name was `BackUpWordPress`
This was causing 404 errors on the JS and CSS files being enqueued with `HMBKP_PLUGIN_SLUG` in the URL. 
It seems this particular environment is case sensitive, maybe not the one you develop on?

you can ignore this commit
https://github.com/pdewouters/BackUpWordPress/commit/15293a7e2ac97d496732ca6dfa1837f6c56ce619