Correct Docker Installation reference to key secret.

[ravindk89]

https://github.com/minio/docs/blob/master/source/tutorials/minio-installation.rst#stable-minio

the _FILE suffix only applies when referencing a Docker secret

Need to update these sections to either call that out, or document both the secret and string.

This is a short-term fix to integrate into the larger rewrite/restructure for the installation docs.

Addresses problem raised in minio/minio#12451

[ravindk89]

7736389 Fixed this a while back, forgot to close the issue.