fix wrong statment in manual.

[arjenzhou]

#694

[codecov-commenter]

Codecov Report

Merging #695 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #695   +/-   ##
=========================================
  Coverage     88.26%   88.26%           
  Complexity      730      730           
=========================================
  Files            69       69           
  Lines          2540     2540           
  Branches        268      268           
=========================================
  Hits           2242     2242           
  Misses          210      210           
  Partials         88       88           

---

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 d6982ad...e35a5f4. Read the comment docs.

[mhall119]

Thanks for the fix @arjenzhou!