s/mean/median/ thanks Julia

IPythonReviewTime.ipynb

@@ -898,7 +898,7 @@
      "cell_type": "markdown",
      "metadata": {},
      "source": [
-      "Based on this, it looks like most review periods are between -4 days (whatever that means) and about two weeks. The mean is (I'm eyeballing this) around 1 day. Not bad at all.\n",
+      "Based on this, it looks like most review periods are between -4 days (whatever that means) and about two weeks. The median is (I'm eyeballing this) around 1 day. Not bad at all.\n",
       "\n",
       "If we look closely at the pull requests outside this range, we might see any number of things: pull requests that are large and hard to understand or pull requests that are blocked by some other task. I don't think pandas would help us much: we'd have to use our brains and look for patterns.\n",
       "\n",