Fix unmatched emphasis tag in git-tutorial

Fix unmatched emphasis tag in git-tutorial

In asciidoc 7.1.2 and prior there is no obvious way to get:

'add'ing

to emphasize only the "add", instead it treats the first apostrophe as the
beginning of an emphasis, and the second apostrophe as a regular
apostrophe and makes the rest of the line an emphasis since there is no
closing apostrophe.  In the newer asciidoc you can do it pretty easily
with __add__ing but I'm not sure it would be best to make that a prereq
for something as silly as this.

Signed-off-by: Andrew Ruder <andy@aeruder.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>