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>

Documentation/MyFirstContribution.txt

@@ -1046,46 +1046,26 @@ directory you specified - you're nearly ready to send out your review!
 [[preparing-cover-letter]]
 === Preparing Email
 
-In addition to an email per patch, the Git community also expects your patches
-to come with a cover letter, typically with a subject line [PATCH 0/x] (where
-x is the number of patches you're sending). Since you invoked `format-patch`
-with `--cover-letter`, you've already got a template ready. Open it up in your
-favorite editor.
+Since you invoked `format-patch` with `--cover-letter`, you've already got a
+cover letter template ready. Open it up in your favorite editor.
 
 You should see a number of headers present already. Check that your `From:`
-header is correct. Then modify your `Subject:` to something which succinctly
-covers the purpose of your entire topic branch, for example:
+header is correct. Then modify your `Subject:` (see <<cover-letter,above>> for
+how to choose good title for your patch series):
 
 ----
-Subject: [PATCH 0/7] adding the 'psuh' command
+Subject: [PATCH 0/7] Add the 'psuh' command
 ----
 
 Make sure you retain the ``[PATCH 0/X]'' part; that's what indicates to the Git
-community that this email is the beginning of a review, and many reviewers
-filter their email for this type of flag.
+community that this email is the beginning of a patch series, and many
+reviewers filter their email for this type of flag.
 
 You'll need to add some extra parameters when you invoke `git send-email` to add
 the cover letter.
 
-Next you'll have to fill out the body of your cover letter. This is an important
-component of change submission as it explains to the community from a high level
-what you're trying to do, and why, in a way that's more apparent than just
-looking at your diff. Be sure to explain anything your diff doesn't make clear
-on its own.
-
-Here's an example body for `psuh`:
-
-----
-Our internal metrics indicate widespread interest in the command
-git-psuh - that is, many users are trying to use it, but finding it is
-unavailable, using some unknown workaround instead.
-
-The following handful of patches add the psuh command and implement some
-handy features on top of it.
-
-This patchset is part of the MyFirstContribution tutorial and should not
-be merged.
-----
+Next you'll have to fill out the body of your cover letter. Again, see
+<<cover-letter,above>> for what content to include.
 
 The template created by `git format-patch --cover-letter` includes a diffstat.
 This gives reviewers a summary of what they're in for when reviewing your topic.