Encourage https for bottle hosting

I assume that we want to encourage people hosting
their own bottles to serve them using `https`.
This PR has a trivial change in the example.
We may consider adding some text to the `root_url`
description with a discussion of security.
Also use https://example.com instead of
http://mikemcquaid.com

 in bottle example.

Closes Homebrew/homebrew#44651.

Signed-off-by: Xu Cheng <xucheng@me.com>

share/doc/homebrew/Bottles.md

@@ -32,7 +32,7 @@ end
 A full example:
 ```ruby
 bottle do
-  root_url "http://mikemcquaid.com"
+  root_url "https://example.com"
   prefix "/opt/homebrew"
   cellar "/opt/homebrew/Cellar"
   revision 4