Update mac.md (#1760)

add a troubleshooting hint

docs/arduino-ide/mac.md

@@ -11,7 +11,7 @@ Installation instructions for Mac OS
   cd esp32 && \
   git submodule update --init --recursive && \
   cd tools && \
-  python get.py
+  python get.py 
   ```
   Where `~/Documents/Arduino` represents your sketch book location as per "Arduino" > "Preferences" > "Sketchbook location" (in the IDE once started). Adjust the command above accordingly if necessary!
   
@@ -20,6 +20,8 @@ Installation instructions for Mac OS
   ```xcrun: error: invalid active developer path (/Library/Developer/CommandLineTools), missing xcrun at: /Library/Developer/CommandLineTools/usr/bin/xcrun```
   
   ```xcode-select --install```
+  
+- 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`
 
 - Restart Arduino IDE