docs: Update test docstrings

Patch docstrings with the following:
- Fix grammar and typo errors.
- Annotate `TODO` for consistency throughout project.

[Note]
TODOs in project adhere to the following style: `@TODO:`

Author: apexDev37

backend/test_flaskr.py

@@ -23,12 +23,12 @@ def setUp(self):
 
 
     def tearDown(self):
-        """Executed after reach test"""
+        """Executed after each test"""
         pass
 
     """
-    TODO
-    Write at least one test for each test for successful operation and for expected errors.
+    @TODO:
+    Write at least one test for each endpoint for successful operation and for expected errors.
     """