Use technical language in README background

Replace promotional phrase "giving you complete control" with
technical description "allowing consumers to implement their own
rendering" in Background section.

Maintains factual tone consistent with documentation standards.

Author: tanem

README.md

@@ -12,7 +12,7 @@
 
 ## Background
 
-This is a React port of [rstacruz](https://github.com/rstacruz)'s [`nprogress`](https://github.com/rstacruz/nprogress) module. It exposes an API that encapsulates the logic of `nprogress` and renders nothing, giving you complete control over rendering.
+This is a React port of [rstacruz](https://github.com/rstacruz)'s [`nprogress`](https://github.com/rstacruz/nprogress) module. It exposes an API that encapsulates the logic of `nprogress` and renders nothing, allowing consumers to implement their own rendering.
 
 ## Usage