Reduce Minio access key minimum length to 3 (#5478)

This is a generic minimum value. The current reason is to support
Azure blob storage accounts name whose length is less than 5. 3 is the
minimum length for Azure.

cmd/server-main.go

@@ -53,7 +53,7 @@ FLAGS:
   {{end}}{{end}}
 ENVIRONMENT VARIABLES:
   ACCESS:
-     MINIO_ACCESS_KEY: Custom username or access key of minimum 5 characters in length.
+     MINIO_ACCESS_KEY: Custom username or access key of minimum 3 characters in length.
      MINIO_SECRET_KEY: Custom password or secret key of minimum 8 characters in length.
 
   BROWSER: