docs: improve formatting in git-send-email documentation

docs: improve formatting in git-send-email documentation

The current documentation for git-send-email had an inconsistent use of
"", ``, and '' for quoting. This commit improves the formatting by
using the same style throughout the documentation. Missing full stops
have also been added at some places.

Finally, the cpan links of necessary perl modules have been added to
make their installation easier.

While at it, the unecessary use of $ with <num> and <int> placeholders
has also been removed.

Signed-off-by: Aditya Garg <gargaditya08@live.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>