Fix doc link in Readme

Author: phil-opp

Readme.md

@@ -1,6 +1,6 @@
 # `volatile`
 
-[![Build Status](https://github.com/rust-osdev/volatile/workflows/Build/badge.svg)](https://github.com/rust-osdev/volatile/actions?query=workflow%3ABuild) [![Docs.rs Badge](https://docs.rs/volatile/badge.svg)](https://docs.rs/spinning_top/)
+[![Build Status](https://github.com/rust-osdev/volatile/workflows/Build/badge.svg)](https://github.com/rust-osdev/volatile/actions?query=workflow%3ABuild) [![Docs.rs Badge](https://docs.rs/volatile/badge.svg)](https://docs.rs/volatile/)
 
 Provides the wrapper type `Volatile`, which wraps a reference to any copy-able type and allows  for volatile memory access to wrapped value. Volatile memory accesses are never optimized away  by the compiler, and are useful in many low-level systems programming and concurrent contexts.