Replace reference from vinum.8 to gvinum.8, it was advised in the PR to replace this with vinum.4, but that's the kernel interface manual, which is not appropriate in my understanding.  I think that gvinum is a suitable replacement for this.
Replace reference from vinum.8 to gvinum.8, it was advised in the PR to
replace this with vinum.4, but that's the kernel interface manual, which
is not appropriate in my understanding.  I think that gvinum is a suitable
replacement for this.

PR:		docs/121938
Submitted by:	"Federico" <federicogalvezdurand at yahoo dot com>
MFC after:	3 days

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