[doc] Fix link in rank demo README.md .

[Sam1301]

No description provided.

[codecov-io]

Codecov Report

Merging #3759 into master will increase coverage by 0.1%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             master    #3759     +/-   ##
===========================================
+ Coverage     51.99%   52.09%   +0.1%     
  Complexity      196      196             
===========================================
  Files           180      180             
  Lines         14307    14350     +43     
  Branches        489      513     +24     
===========================================
+ Hits           7439     7476     +37     
- Misses         6634     6637      +3     
- Partials        234      237      +3

Impacted Files	Coverage Δ	Complexity Δ
.../scala/ml/dmlc/xgboost4j/scala/spark/XGBoost.scala	77.12% <0%> (+1.45%)	0% <0%> (ø)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9e73087...39adc7e. Read the comment docs.

[hcho3]

Thanks!