Documentation: use 8-space tabs with Asciidoctor

Documentation: use 8-space tabs with Asciidoctor

Asciidoctor expands tabs at the beginning of a line.  However, it does
not expand them into 8 spaces by default.  Since we use 8-space tabs,
tell Asciidoctor that we want 8 spaces by setting the tabsize attribute.
This ensures that our ASCII art renders properly.

Signed-off-by: brian m. carlson <redacted>
Signed-off-by: Junio C Hamano <redacted>