Fix missing space in order statistics warning message

[ishaan-arora-1]

Fixes #329

R's warning() concatenates string arguments with no separator, so the two sentences in the loo_order_stat_check() warning were getting glued together as "chance.See".

Added the missing trailing space after the first sentence.

Also tightened the existing test to check the full message text.

[jgabry]

Thanks! Looks good. Merging now.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.78%. Comparing base (2883f25) to head (7f66ea0).
⚠️ Report is 16 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #330   +/-   ##
=======================================
  Coverage   92.78%   92.78%           
=======================================
  Files          31       31           
  Lines        2992     2992           
=======================================
  Hits         2776     2776           
  Misses        216      216           

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