Fix another typo in the timeseries documentation

[adamjstewart]

Just noticed this right after #15666 was merged.

[codecov-io]

Codecov Report

Merging #15667 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #15667      +/-   ##
==========================================
- Coverage   91.03%   91.01%   -0.03%     
==========================================
  Files         143      143              
  Lines       49382    49382              
==========================================
- Hits        44957    44944      -13     
- Misses       4425     4438      +13

Impacted Files	Coverage Δ
pandas/io/gbq.py	25% <0%> (-58.34%)	❌
pandas/tools/merge.py	93.61% <0%> (-0.34%)	❌
pandas/util/testing.py	80.7% <0%> (-0.19%)	❌
pandas/core/frame.py	97.87% <0%> (-0.1%)	❌

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[jreback]

thanks!