[doc] secret_key: <access key> should be <secret key>

Closes elastic#168. (cherry picked from commit 3a4e472) (cherry picked from commit ff6b23b)

Author: jerryleooo

README.md

@@ -268,7 +268,7 @@ repositories:
  private-bucket:
  bucket: <bucket not accessible by default key>
  access_key: <access key>
- secret_key: <access key>
+ secret_key: <secret key>
  remote-bucket:
  bucket: <bucket in other region>
  region: <region>