searching.asc: mention that "git grep" can search the index as well

Signed-off-by: Robert P. J. Day <[email protected]>

Author: robertpjday

book/07-git-tools/sections/searching.asc

@@ -8,7 +8,7 @@ We'll go through a few of them.
 [[_git_grep]]
 ==== Git Grep
 
-Git ships with a command called `grep` that allows you to easily search through any committed tree or the working directory for a string or regular expression.
+Git ships with a command called `grep` that allows you to easily search through any committed tree, the working directory or even the index for a string or regular expression.
 For the examples that follow, we'll search through the source code for Git itself.
 
 By default, `git grep` will look through the files in your working directory.