Change all master branch references to main. (#67)

This affects just the GHAs.

Author: sarenameas

.github/workflows/ci.yml

@@ -3,7 +3,7 @@ on:
   push:
     branches: ["**"]
   pull_request:
-    branches: [master]
+    branches: [main]
   workflow_dispatch:
 jobs:
   unittest:
@@ -79,7 +79,7 @@ jobs:
         - name: Checkout Parent Repo
           uses: actions/checkout@v2
           with:
-            ref: master
+            ref: main
             repository: aws/aws-iot-device-sdk-embedded-C
         - run: rm -rf libraries/standard/coreJSON
         - name: Clone This Repo