Add missing definition of the 'w' variable (#1434)

Author: chrishtr

book/chrome.md

@@ -137,6 +137,7 @@ method for that:
 class BlockLayout:
     def word(self, node, word):
         # ...
+        w = font.measure(word)
         if self.cursor_x + w > self.width:
             self.new_line()
 ```