send-email: simplify Gmail example in the documentation

send-email: simplify Gmail example in the documentation

There is no need for use to manually call ‘git credential’ especially
as the interface isn’t super user-friendly and a bit confusing.  ‘git
send-email’ will do that for them at the first execution and if the
password matches, it will be saved in the store.

Simplify the documentaion so it dosn’t include the ‘git credential’
invocation (which was incorrect anyway as it should use ‘approve’
instead of ‘fill’) and instead just mentions that credentials helper
must be set up.

Signed-off-by: Michał Nazarewicz <mina86@mina86.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>