Fix period = 1 in evaluation monitor.

[trivialfis]

No description provided.

[codecov-io]

Codecov Report

Merging #6441 (bd79b2f) into master (956beea) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #6441      +/-   ##
==========================================
- Coverage   79.97%   79.94%   -0.03%     
==========================================
  Files          12       12              
  Lines        3475     3475              
==========================================
- Hits         2779     2778       -1     
- Misses        696      697       +1     

Impacted Files	Coverage Δ
python-package/xgboost/callback.py	79.70% <100.00%> (-0.25%)	⬇️

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