[doc] Fix early stopping document.

[trivialfis]

Close #7333 .

[codecov-commenter]

Codecov Report

Merging #7334 (59601e4) into master (fbb0dc4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #7334   +/-   ##
=======================================
  Coverage   83.68%   83.68%           
=======================================
  Files          13       13           
  Lines        3885     3885           
=======================================
  Hits         3251     3251           
  Misses        634      634           

Impacted Files	Coverage Δ
python-package/xgboost/training.py	94.97% <ø> (ø)

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[hcho3]

Thanks!