doc: fix double "See" in front of reference

* doc/gcrypt.texi: Change @xref to @ref when text already has 'see' in
the front.

@xref references start with `See ...'. Use @ref instead
when text already has 'see' in front.

(cherry picked from commit bd75f0e89817b5708c57efab49e3eb4e035186e2)

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>