Update rebasing.asc

Tiny typo

Git calculate a checksum -> Git calculates a checksum

Author: rahrah

book/03-git-branching/sections/rebasing.asc

@@ -181,7 +181,7 @@ It's pretty safe to assume that the other developer doesn't want `C4` and `C6` t
 
 If you *do* find yourself in a situation like this, Git has some further magic that might help you out. If someone on your team force pushes changes that overwrite work that you've based work on, your challenge is to figure out what is yours and what they've rewritten.
 
-It turns out that in addition to the commit SHA checksum, Git also calculate a checksum that is based just on the patch introduced with the commit. This is called a ``patch-id''.
+It turns out that in addition to the commit SHA checksum, Git also calculates a checksum that is based just on the patch introduced with the commit. This is called a ``patch-id''.
 
 If you pull down work that was rewritten and rebase it on top of the new commits from your partner, Git can often successfully figure out what is uniquely yours and apply them back on top of the new branch.