Fix PIDE docs

[termi-official]

https://docs.sciml.ai/MethodOfLines/stable/tutorials/PIDE/ is broken. This fixes it.

[codecov]

Codecov Report

Merging #262 (0b3ea2a) into master (7e046c4) will increase coverage by 1.47%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #262      +/-   ##
==========================================
+ Coverage   77.13%   78.60%   +1.47%     
==========================================
  Files          40       40              
  Lines        2038     2038              
==========================================
+ Hits         1572     1602      +30     
+ Misses        466      436      -30     

see 2 files with indirect coverage changes

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[xtalax]

Thanks looks good