add missing porosity to PorousMediaBurner.py

[g3bk47]

The chemical source term of the gas-phase temperature equation in the PorousMediaBurner.py example was missing the porosity. The actual code in the supplementary material of the cited paper handles this correctly, but the porosity got lost while cleaning up the code for this example case.

[codecov[bot]]

Codecov Report

Merging #1617 (ba5d97b) into main (c1281c1) will increase coverage by 0.01%. The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1617      +/-   ##
==========================================
+ Coverage   72.65%   72.66%   +0.01%     
==========================================
  Files         370      370              
  Lines       56255    56255              
  Branches    20353    20353              
==========================================
+ Hits        40873    40880       +7     
+ Misses      12379    12372       -7     
  Partials     3003     3003              

see 1 file with indirect coverage changes

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