Update mac.md (#2745)

Author: dash0820

Diff for: docs/arduino-ide/mac.md

@@ -23,5 +23,7 @@ Installation instructions for Mac OS
 
 - Try `python3` instead of `python` if you get the error: `IOError: [Errno socket error] [SSL: TLSV1_ALERT_PROTOCOL_VERSION] tlsv1 alert protocol version (_ssl.c:590)` when running `python get.py`
 
+- If you get the following error when running `python get.py` urllib.error.URLError: <urlopen error SSL: CERTIFICATE_VERIFY_FAILED, go to Macintosh HD > Applications > Python3.6 folder (or any other python version), and run the following scripts: Install Certificates.command and Update Shell Profile.command
+
 - Restart Arduino IDE