Small fixes to make clippy happy

[jondo2010]

Our project Rerun uses pretty strict clippy settings, and I ran into several linting issues with the code generated by shadow-rs:

error: item in documentation is missing backticks
    --> /Users/john/Source/rerun/target/debug/build/rerun-5295388bed2c865d/out/shadow.rs:1254:67
     |
1254 | /// The common version const. It's so easy to use this const with CLAP_VERSION.
     |                                                                   ^^^^^^^^^^^^
     |
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#doc_markdown
     = note: `-D clippy::doc-markdown` implied by `-D warnings`

and

error: missing documentation for a module
  --> crates/rerun/src/lib.rs:94:1
   |
94 | shadow_rs::shadow!(build);
   | ^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `-D missing-docs` implied by `-D warnings`

This PR fixes those lints.

[baoyachi]

@jondo2010 Thx. please use shadow-rs new version:

shadow-rs = "0.20.1"