Fix minor mistakes in docs

[carstenbauer]

• Fix: y -> log(y)^7 needed to be j -> log(x[j])^7
• Added local keywords for safety/good measure. (It doesn't matter for the specific examples but more general cases.)

[codecov[bot]]

Codecov Report

Merging #16 (3f5d506) into main (d424582) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #16   +/-   ##
=======================================
  Coverage   96.66%   96.66%           
=======================================
  Files           1        1           
  Lines          60       60           
=======================================
  Hits           58       58           
  Misses          2        2           

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[lmiq]

Thanks!