Fix subscript typo in docs

[Christopher-Bradshaw]

[codecov[bot]]

Codecov Report

Merging #278 into master will increase coverage by 0.06%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #278      +/-   ##
==========================================
+ Coverage   95.21%   95.28%   +0.06%     
==========================================
  Files          22       22              
  Lines        3178     3222      +44     
==========================================
+ Hits         3026     3070      +44     
  Misses        152      152              

Impacted Files	Coverage Δ
pygam/tests/test_terms.py	100.00% <0.00%> (ø)
pygam/pygam.py	94.80% <0.00%> (+<0.01%)	:arrow_up:
pygam/tests/test_utils.py	96.55% <0.00%> (+0.04%)	:arrow_up:
pygam/callbacks.py	95.23% <0.00%> (+0.07%)	:arrow_up:
pygam/terms.py	94.60% <0.00%> (+0.19%)	:arrow_up:
pygam/distributions.py	88.88% <0.00%> (+0.48%)	:arrow_up:
pygam/tests/conftest.py	98.21% <0.00%> (+0.65%)	:arrow_up:

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