Fix :rtype: directive in ProjectColumn docs

[dhruvkb]

This PR fixes a missing colon after :rtype in the ProjectColumn docs which was causing the documentation to misrender.

[codecov-commenter]

Codecov Report

Merging #1927 (670d1f7) into master (babcbcd) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1927   +/-   ##
=======================================
  Coverage   98.86%   98.86%           
=======================================
  Files         108      108           
  Lines       11060    11060           
=======================================
  Hits        10934    10934           
  Misses        126      126           

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