Fix lingering Markdown for git-send-email.io

[noelleleigh]

• [x] I provide my work under the project license.
• [x] I grant such license of my work as is required for the purposes of future print editions to Ben Straub and Scott Chacon.

Changes

• Fix the link to https://git-send-email.io to use AsciiDoc syntax instead of Markdown.

Context

Builds on #1489

[HonkingGoose]

@noelleleigh Good catch! I got my Markdown and my Asciidoctor link format mixed up... 🙃

I like the window _blank property you're using with the caret!

Quote from Asciidoctor writers guide, links and images:

Since _blank is the most common window name, we’ve introduced shorthand for it. Just end the link text with a caret (^):

Let's view the raw HTML of the link:view-source:asciidoctor.org[Asciidoctor homepage^].

Maybe we should use that by default, so that the ProGit 2 book stays open in the original tab whenever you click on an external link.