MyFirstContribution: clarify asciidoc dependency

MyFirstContribution: clarify asciidoc dependency

Per IRC:

[19:52] <lkmandy> With respect to the MyFirstContribution tutorial, I
will like to suggest this - Under the section "Adding Documentation",
just before the "make all doc" command, it will be really helpful to
prompt a user to check if they have the asciidoc package installed, if
they don't, the command should be provided or they can just be pointed
to install it

So, let's move the note about the dependency to before the build command
blockquote.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>