Update docs link in Cargo.toml to HTTPS version

Currently, the documentation link is specified in Cargo.toml as:

documentation = "http://rust-lang.github.io/mdBook/index.html"

which propagates to crates.io and if users click on the docs link there they get the non-TLS version. Not likely to cause major issues, but still best practice to use the HTTPS version since it's there

Author: Benjins

Cargo.toml

@@ -6,7 +6,7 @@ authors = [
     "Michael-F-Bryan <[email protected]>",
     "Matt Ickstadt <[email protected]>"
 ]
-documentation = "http://rust-lang.github.io/mdBook/index.html"
+documentation = "https://rust-lang.github.io/mdBook/index.html"
 edition = "2021"
 exclude = ["/guide/*"]
 keywords = ["book", "gitbook", "rustbook", "markdown"]