message: doc: capitalization change for consistency.
diff: * [Cargo.toml vs Cargo.lock](guide/cargo-toml-vs-cargo-lock.md)
  * [Tests](guide/tests.md)
  * [Continuous Integration](guide/continuous-integration.md)
  - * [Cargo home](guide/cargo-home.md)
  + * [Cargo Home](guide/cargo-home.md)
  * [Build Cache](guide/build-cache.md)
  * [Cargo Reference](reference/index.md)
  