updated docs ...

...to include YouTube link and optimize doc links.

pfas-tree/PFAS_Tree.Rmd

@@ -65,8 +65,9 @@ and [help](https://pubchem.ncbi.nlm.nih.gov/classification/docs/classification_h
 pages, or by reaching out to
 [pubchem-help@ncbi.nlm.nih.gov](mailto:pubchem-help@ncbi.nlm.nih.gov) 
 for more information.
-
-Finally, an interactive walkthrough is available on the 
+Finally, an interactive walkthrough of the 
+[PubChem PFAS Tree](https://pubchem.ncbi.nlm.nih.gov/classification/#hid=120)
+featuring Emma Schymanski and Hans Peter Arp is available on the 
 [ZeroPM YouTube Channel](https://www.youtube.com/watch?v=g-sAazaagas).
 <!-- - https://pubchem.ncbi.nlm.nih.gov/classification/ -->
 <!-- - https://pubchemdocs.ncbi.nlm.nih.gov/classification-browser  -->