Fix git submodule command in CONTRIBUTING.md

This typo dates back to de60ed2

Author: jmikola

CONTRIBUTING.md

@@ -209,13 +209,13 @@ git checkout 2.1.0
 During development, it may be necessary to temporarily point the submodule to a
 commit on the developer's fork. For instance, the developer may be working on a
 PHP driver feature that depends on unmerged or unreleased changes. In this case,
-the submodule path can be updated using the `git submodules set-url` command can
-be used to change the URL, and `git submodules set-branch` can be used to point
+the submodule path can be updated using the `git submodule set-url` command can
+be used to change the URL, and `git submodule set-branch` can be used to point
 the submodule to a development branch:
 
 ```shell
-git submodules set-url src/libmongoc https://github.com/<owner>/<repo>.git
-git submodules set-branch -b <branch> src/libmongoc
+git submodule set-url src/libmongoc https://github.com/<owner>/<repo>.git
+git submodule set-branch -b <branch> src/libmongoc
 ```
 
 #### Ensure version information is correct (libmongocrypt only)