Change the first lowercase tween to a capitalised Tween to match previous code. (#3275)

The capitalised Tween is a class name and the lowercase tween is a variable name.
While this works, the initial presentation of the call uses the classname for both interpolation options.
This is being picky but is accurate.

Co-authored-by: sambler <[email protected]>

getting_started/step_by_step/ui_code_a_life_bar.rst

@@ -440,7 +440,7 @@ duration in seconds.
 
 ::
 
-    ...  0.6, tween.TRANS_LINEAR, Tween.EASE_IN)
+    ...  0.6, Tween.TRANS_LINEAR, Tween.EASE_IN)
 
 The last two arguments are constants from the ``Tween`` class.
 ``TRANS_LINEAR`` means the animation should be linear. ``EASE_IN``