Update to Board Manager URL

The repo switch to the main branch convention, which changes the URL to the board manager JSON file. GitHub caches the previous (master branch) version, so nothing broke, but new updates after the change aren’t available to anyone using the old URL.

Author: stonehippo

README.md

@@ -55,7 +55,7 @@ _These boards are supported by [Espressif](https://github.com/espressif/arduino-
 
 To add board support for our products, start Arduino and open the Preferences window (**File** > **Preferences**). Now copy and paste the following URL into the 'Additional Boards Manager URLs' input field:
 
-	https://raw.githubusercontent.com/sparkfun/Arduino_Boards/master/IDE_Board_Manager/package_sparkfun_index.json
+	https://raw.githubusercontent.com/sparkfun/Arduino_Boards/main/IDE_Board_Manager/package_sparkfun_index.json
 
 ![Location of Additional Boards Manager URL input field](prefs-arrow.png)