URL-encode link to serial interval paper

tutorial.ipynb

@@ -171,7 +171,7 @@
     "\n",
     "The implicit assumption in the generative process above is that an infected person is only infectious for a single day and that it then takes just one day to infect other people.\n",
     "\n",
-    "In reality, the time it takes for the primary person to infect others follows a distribution. They might infect one person the next day, two the day after etc. This delay distribution is officially known as the \"*generation time*\" and we will model it with a probability distribution from [this study](https://www.ijidonline.com/article/S1201-9712(20)30119-3/pdf) (the study actually provides an estimate for something closely related to the generation time but it's fine to use it for this)."
+    "In reality, the time it takes for the primary person to infect others follows a distribution. They might infect one person the next day, two the day after etc. This delay distribution is officially known as the \"*generation time*\" and we will model it with a probability distribution from [this study](https://www.ijidonline.com/article/S1201-9712%2820%2930119-3/pdf) (the study actually provides an estimate for something closely related to the generation time but it's fine to use it for this)."
    ]
   },
   {