update CDC link

Author: afeld

final_project/lead.ipynb

@@ -1261,7 +1261,7 @@
    "metadata": {},
    "source": [
     "**Citywide Line Chart**  \n",
-    "The above line chart shows how elevated BLL rates have fallen over the years, consistent with my hypothesis. As there are a relatively few number of children with elevated BLL >=10 or >=15 µg/dL per 1000, I chose to proceed with the majority of my analysis using the  BLL >=5 µg/dL rate per 1000 tested. Additionally the [CDC reference](https://www.cdc.gov/nceh/lead/prevention/blood-lead-levels.htm) for lead exposure is 3.5µg/dL and thus using >=5µg/dL is adequate for analysing elevated BLL for children across the city."
+    "The above line chart shows how elevated BLL rates have fallen over the years, consistent with my hypothesis. As there are a relatively few number of children with elevated BLL >=10 or >=15 µg/dL per 1000, I chose to proceed with the majority of my analysis using the  BLL >=5 µg/dL rate per 1000 tested. Additionally the [CDC reference](https://www.cdc.gov/lead-prevention/testing/) for lead exposure is 3.5µg/dL and thus using >=5µg/dL is adequate for analysing elevated BLL for children across the city."
    ]
   },
   {