Update chapter3-exercise-centered-text-tests.md

Fixed missing doctest output normalization

tests/chapter3-exercise-centered-text-tests.md

@@ -81,7 +81,7 @@ You should be able to mix centered and normal text on different lines
 
 Ensure that the application correctly handles the transition from centered to regular text within the same sequence
 
-    >>> test_layout('<h1 class="title">center this</h1>then this is regular')
+    >>> test_layout('<h1 class="title">center this</h1>then this is regular') #doctest: +NORMALIZE_WHITESPACE   
     [(312.0, 21.0, 'center', Font size=16 weight=normal slant=roman style=None),
      (424.0, 21.0, 'this', Font size=16 weight=normal slant=roman style=None), 
      (13.0, 41.0, 'then', Font size=16 weight=normal slant=roman style=None),