mime: fix mime type file name on Plan 9

mime: fix mime type file name on Plan 9

There was a typo introduced in the initial
implementation of the Plan 9 support of
the mime package.

On Plan 9, the mime type file name should be
/sys/lib/mimetype instead of /sys/lib/mimetypes.

Change-Id: If0f0a9b6f3fbfa8dde551f790e83bdd05e8f0acb
Reviewed-on: https://go-review.googlesource.com/23087
Run-TryBot: Minux Ma <minux@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>