Fix docs typo modules_intro.rst (#40835) (#43452)

Fixed typo in modules_intro.rst.

+label: docsite_pr
(cherry picked from commit f785bf3a9c)

docs/docsite/rst/user_guide/modules_intro.rst

@@ -3,7 +3,7 @@
 Introduction
 ============
 
-Modules (also referred to as "task plugins" or "library plugins") are discrete unites of code that can be used from the command line or in a playbook task.
+Modules (also referred to as "task plugins" or "library plugins") are discrete units of code that can be used from the command line or in a playbook task.
 
 Let's review how we execute three different modules from the command line::