Fix trailing whitespace in theme documentation

Author: Revisto

doc/usage/theming.rst

@@ -193,9 +193,9 @@ These themes are:
     .. versionadded:: 3.2
 
   - **is_rtl** (true or false): Enable right-to-left (RTL) text direction.
-    Use this for languages that are read right-to-left (like Farsi, Arabic, 
+    Use this for languages that are read right-to-left (like Farsi, Arabic,
     Hebrew, etc). When enabled, the theme will:
-    
+
     - Change text direction to RTL
     - Mirror layout components
     - Keep code blocks in LTR direction