ch-10: avoid unnecessary mention update-index's --add

The book clearly mentions that --add needs to be passed to 'git update-index'
only when the file is new to the index. When the second version of 'test.txt' is
added, the index know about the file already.

So, avoid passing the '--add' when adding the second version of 'test.txt'

Author: sivaraam

book/10-git-internals/sections/objects.asc

@@ -202,7 +202,7 @@ You'll now create a new tree with the second version of `test.txt` and a new fil
 [source,console]
 ----
 $ echo 'new file' > new.txt
-$ git update-index --add --cacheinfo 100644 \
+$ git update-index --cacheinfo 100644 \
   1f7a7a472abf3dd9643fd615f6da379c4acb3e3a test.txt
 $ git update-index --add new.txt
 ----