doc: Change statistics function xref to the right target

doc: Change statistics function xref to the right target

Commit 7d3b7011b added a link to the statistics functions, which at the
time were anchored under the section for statistics views.  aebe989477a
added a separate section for statistics functions, but the link was not
updated to point to the new anchor.  Fix by changing the xref.

Backpatch to all supported branches.

Author: Peter Smith <peter.b.smith@fujitsu.com>
Discussion: https://postgr.es/m/CAHut+Ptr0jKzNNtWnssLq+3jNhbyaBseqf6NPrWHk08mQFRoTg@mail.gmail.com
Backpatch-through: 11