Add a period to the end of a list line (#33351)

Co-authored-by: Alex Nguyen <[email protected]>

Author: SeanAverS

content/get-started/using-git/about-git-subtree-merges.md

@@ -19,7 +19,7 @@ Typically, a subtree merge is used to contain a repository within a repository.
 
 The best way to explain subtree merges is to show by example. We will:
 
-- Make an empty repository called `test` that represents our project
+- Make an empty repository called `test` that represents our project.
 - Merge another repository into it as a subtree called `Spoon-Knife`.
 - The `test` project will use that subproject as if it were part of the same repository.
 - Fetch updates from `Spoon-Knife` into our `test` project.