[Fix] Update the FCOS3D model link in the Demo doc

[Tai-Wang]

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Motivation

Update the FCOS3D model link in the Demo doc.

[codecov]

Codecov Report

Merging #818 (744ce33) into master (31ef559) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #818   +/-   ##
=======================================
  Coverage   49.07%   49.07%           
=======================================
  Files         210      210           
  Lines       15961    15961           
  Branches     2547     2547           
=======================================
  Hits         7833     7833           
  Misses       7626     7626           
  Partials      502      502           

Flag	Coverage Δ
unittests	49.07% <ø> (ø)

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