Restores missing step in objects.asc

Restores missing step: update cache to point to "new" test.txt (`1f7a7a472abf3dd9643fd615f6da379c4acb3e3a`) when building the tree `0155eb4229851634a0f03eb265b69f5a2d56f341`

Author: flyingzumwalt

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

@@ -188,6 +188,8 @@ 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 --cacheinfo 100644 \
+  1f7a7a472abf3dd9643fd615f6da379c4acb3e3a test.txt
 $ git update-index test.txt
 $ git update-index --add new.txt
 ----