config: Drop 'type' from bind-mount example

This initially came in with b3918a2 (Add bind mount example,
2015-06-30), but the 'bind' value is not one of the types you can get
out of /proc/filesystems.  Instead, bind mounts should leave the type
unset and put either 'bind' or 'rbind' in options (although neither of
those are documented either [1]).  Since documenting (r)bind seems to
be too difficult, we should at least stop setting type in the example
to stop confusing users [2,3].

Runc still checks .Type instead of .Options for bind-ness in a few
places [4,5,6], but we can address all of those by setting .Device to
"bind" depending on .Options at [4].

[1]: opencontainers#771
[2]: opencontainers/runc#1744
[3]: https://groups.google.com/a/opencontainers.org/forum/#!topic/dev/2gia6t1Dnv0
     Subject: Runc default mount type
     Date: Tue, 6 Mar 2018 14:19:40 -0800 (PST)
     Message-Id: <[email protected]>
[4]: https://github.com/opencontainers/runc/blob/v1.0.0-rc5/libcontainer/specconv/spec_linux.go#L272-L276
[5]: https://github.com/opencontainers/runc/blob/v1.0.0-rc5/libcontainer/rootfs_linux.go#L33
[6]: https://github.com/opencontainers/runc/blob/v1.0.0-rc5/libcontainer/rootfs_linux.go#L234

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

config.md

@@ -115,7 +115,6 @@ For POSIX platforms the `mounts` structure has the following fields:
     },
     {
         "destination": "/data",
-        "type": "bind",
         "source": "/volumes/testing",
         "options": ["rbind","rw"]
     }