Update README.MD

Update to Board Manager URL to new branch naming convention (e.g. `master` to `main` ), similar to the pull request changes that were committed in the SparkFun Arduino Boards repo => sparkfun/Arduino_Boards#91 (comment) .

No need to update the version number for the Arduino Apollo3 Core since these changes are just in the README.md.

Author: bboyho

README.MD

@@ -21,8 +21,8 @@ An mbed-os enabled Arduino core for Ambiq Apollo3 based boards
 ## Installation
 ### **Arduino Boards Manager** (recommended)
   - Copy / paste the JSON package URL into the 'Additional Boards Manager URLs' field in Arduino preferences
-    ```https://raw.githubusercontent.com/sparkfun/Arduino_Apollo3/master/package_sparkfun_apollo3_index.json```
-    [https://raw.githubusercontent.com/sparkfun/Arduino_Apollo3/master/package_sparkfun_apollo3_index.json](https://raw.githubusercontent.com/sparkfun/Arduino_Apollo3/master/package_sparkfun_apollo3_index.json)
+    ```https://raw.githubusercontent.com/sparkfun/Arduino_Apollo3/main/package_sparkfun_apollo3_index.json```
+    [https://raw.githubusercontent.com/sparkfun/Arduino_Apollo3/main/package_sparkfun_apollo3_index.json](https://raw.githubusercontent.com/sparkfun/Arduino_Apollo3/main/package_sparkfun_apollo3_index.json)
   - Open 'Boards Manager' and select 'SparkFun Apollo3 Boards' - install the latest version
 
 ### **Git** (development)