MyFirstContribution: reference "The cover letter" in "Preparing Email"

MyFirstContribution: reference "The cover letter" in "Preparing Email"

The previous commit added a standalone section on the purpose of the
cover letter, drawing inspiration from the existing content of the
"Preparing Email" section.

Adjust "Preparing Email" to reference "The cover letter", to avoid
content duplication.

Also, use the imperative mode for the cover letter subject, as is done
in "The cover letter".

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>