Use “their” instead of “his”

So that no gender is implied in this sentence and the explanation makes sense for people of all genders.

Author: hidde

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

@@ -376,7 +376,7 @@ Submodule DbConnector c3f01dc..c87d55d:
 Git will by default try to update *all* of your submodules when you run `git submodule update --remote` so if you have a lot of them, you may want to pass the name of just the submodule you want to try to update.
 
 ===== Pulling Upstream Changes from the Project Remote
-Let's now step into the shoes of your collaborator, who has his own local clone of the MainProject repository.
+Let's now step into the shoes of your collaborator, who has their own local clone of the MainProject repository.
 Simply executing `git pull` to get your newly committed changes is not enough:
 
 [source,console]