Correct MLINKS for sbuf_hexdump(9)

sbuf_hexdump(9) should be linked to sbuf(9), not hexdump(3). Another
review will be posted to deduplicate the sbuf_hexdump reference in
in hexdump(3) or at the very least make the information less duplicative.

MFC after: 1 week
X-MFC with: r313437
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