typo issue fix

Author: AjitPadhi-Microsoft

src/backend/common/config/app_config.py

@@ -177,7 +177,7 @@ def _get_optional(self, name: str, default: str = "") -> str:
             default: Default value if not found (default: "")
 
         Returns:
-            The value of the environment variable or the default value.
+            The value of the environment variable or the default value
         """
         if name in os.environ:
             return os.environ[name]