xen/include/public: fix 9pfs xenstore path description

In xen/include/public/io/9pfs.h the name of the Xenstore backend node
"security-model" should be "security_model", as this is how the Xen
tools are creating it and qemu is reading it.

Fixes: ad58142e73 ("xen/public: move xenstore related doc into 9pfs.h")
Fixes: cf1d2d22fd ("docs/misc: Xen transport for 9pfs")
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jason Andryuk <jandryuk@gmail.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>

xen/include/public/io/9pfs.h

@@ -64,7 +64,7 @@
  *
  *         Host filesystem path to share.
  *
- *    security-model
+ *    security_model
  *         Values:         "none"
  *
  *         *none*: files are stored using the same credentials as they are