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>

Documentation/tutorial.txt

@@ -111,7 +111,7 @@ make it real.
 Note: don't forget to 'add' a file again if you modified it after the
 first 'add' and before 'commit'. Otherwise only the previous added
 state of that file will be committed. This is because git tracks
-content, so what you're really 'add'ing to the commit is the *content*
+content, so what you're really 'adding' to the commit is the *content*
 of the file in the state it is in when you 'add' it.
 
 2) By using 'git commit -a' directly