Improve grammar
Do two minor grammar improvements.
/// a continous work in progress, and can be incorrect
/// or stale.
///
-/// Writing a high quality documentation is
+/// Writing high quality documentation is
/// a lot of work. Please help us! If you spot any
-/// mistakes or ways to improve it:
+/// mistakes or ways to make improvements:
///
/// 1. Open
/// [user documentation source code directory](https://github.com/crev-dev/cargo-crev/tree/master/cargo-crev/src/doc),
