Quick fixes for formula typos

[pjk645-zz]

This is in reference to #262

I changed the formulas to be single variable functions. I'm not sure if the previous intent was to imply the existence of the two variable 'tensor' functions. If that is in fact the case, then separating the two variable functions out from the single variable would be easier to read:
$$\beta_0 + f_1(X_1) + \ldots + f_M(X_M) + \sum_{j=0}^{M} \sum_{k\neq j} f_{j,k}(X_j, X_k)$$
where the double indices imply the dependance on two variables. It should also probably be mentioned that having the two variable functionality is generalizing beyond your typical GAM, which I believe are only supposed to be single variable functions

[codecov]

Codecov Report

Merging #263 into master will increase coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #263      +/-   ##
==========================================
+ Coverage   95.05%   95.18%   +0.12%     
==========================================
  Files          22       22              
  Lines        3178     3178              
==========================================
+ Hits         3021     3025       +4     
+ Misses        157      153       -4

Impacted Files	Coverage Δ
pygam/utils.py	87.73% <0%> (+0.3%)	⬆️
pygam/tests/test_GAM_methods.py	100% <0%> (+0.36%)	⬆️
pygam/tests/test_utils.py	96.5% <0%> (+1.39%)	⬆️

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