doc: convert git-grep synopsis and options to new style

doc: convert git-grep synopsis and options to new style

Convert git-grep.adoc from [verse]/single-quote style to the modern
synopsis-block style:

- Replace [verse] with [synopsis] in SYNOPSIS block
- Change 'git grep' to git grep (no single quotes)
- Backtick-quote all OPTIONS terms
- Convert inline man page refs: grep(1) -> `grep`(1)
- Convert inline command refs: 'git diff' -> `git diff`
- Convert prose placeholders: <file> -> _<file>_

Signed-off-by: Jean-Noël Avila <jn.avila@free.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>