Fix links in doc

[dengelt]

When building the docs, cargo throws some warnings:

warning: this URL is not a hyperlink
  --> src/lib.rs:15:74
   |
15 | //! * `bdays::calendars::brazil::BrazilExchange` : B3 Exchange holidays (http://www.b3.com.br)
   |                                                                          ^^^^^^^^^^^^^^^^^^^^ help: use an automatic link instead: `<http://www.b3.com.br>`
   |
   = note: bare URLs are not automatically turned into clickable links
   = note: `#[warn(rustdoc::bare_urls)]` on by default

warning: this URL is not a hyperlink
  --> src/calendars/brazil.rs:80:27
   |
80 | /// B3 Exchange holidays (http://www.b3.com.br).
   |                           ^^^^^^^^^^^^^^^^^^^^ help: use an automatic link instead: `<http://www.b3.com.br>`
   |
   = note: bare URLs are not automatically turned into clickable links

I implemented cargo's suggestions and changed the protocol from http to https.

[felipenoris]

Thanks!