Update Countdown.ipynb

Countdown.ipynb

@@ -20,7 +20,7 @@
     "\n",
     "> Fill in the blanks so that this equation makes arithmetical sense:\n",
     "\n",
-    "> 10 □  9 □  8 □  7 □  6 □  5 □ 4 □  3 □  2 □  1 = 2016\n",
+    "> `10 _ 9 _ 8 _ 7 _ 6 _ 5 _ 4 _ 3 _ 2 _ 1 = 2016`\n",
     "\n",
     "> You are allowed to use *only* the four basic arithmetical operations: +, -, ×, ÷. But brackets (parentheses) can be used wherever needed. So, for example, the solution could begin\n",
     "\n",