Fix Bug #1: Blank board

There are actually 2 bugs in this program:

1. We return from the function inside the first for-loop. This
means that the function returns after a single pass through the
loop, before we've had a chance to build up our full board. We
fix this bug by moving the return statement outside of both
loops.

2. We don't reset the `row` variable after each time through
the first for-loop. This means that we keep adding to the same
list, instead of splitting our adding between multiple different
rows. We fix this bug by moving `row = []` inside the first
for-loop so that we reset the row each time.

editions/6-debugging/1-blank-board.py

@@ -10,12 +10,12 @@ def blank_board(width, height):
     * https://robertheaton.com/2018/10/09/programming-projects-for-advanced-beginners-3-a/
     """
     board = []
-    row = []
     for _ in range(height):
+        row = []
         for _ in range(width):
             row.append(None)
         board.append(row)
-        return board
+    return board
 
 if __name__ == "__main__":
     test_board = blank_board(4, 3)