Change platform and update docs in file

Author: saville

examples/build/secrets/platforms-buildrunner.yaml

@@ -1,7 +1,6 @@
 # In order to use secrets, you need to set use-legacy-builder to false in the config file OR
 # add platforms to the build section
-# To run this example, you need to set the SECRET_PASSWORD environment variable
-# and run the example with the following command:
+# To run this example, use the following command:
 # SECRET2=my_secret ./run-buildrunner.sh -f examples/build/secrets/platforms-buildrunner.yaml
 # More info about secrets: https://docs.docker.com/build/building/secrets/
 steps:
@@ -26,4 +25,4 @@ steps:
         - id=secret2,env=SECRET2
       platforms:
         - linux/amd64
-        - linux/arm64
+        - linux/arm64/v8