Updating FreeRTOS branch

Change-Id: I44e6b23dd5a8fa176e77a0dec64d770729599c7d
diff --git a/externals.json b/externals.json
index d06dc84..ea992a4 100644
--- a/externals.json
+++ b/externals.json
@@ -27,7 +27,7 @@
         {
             "path": "core_software/rtos/freertos",
             "fetchurl": "https://github.com/aws/amazon-freertos.git",
-            "revision": "master"
+            "revision": "main"
         },
         {
             "path": "core_software/rtos/freertos/freertos_kernel",