Update model text in rep agent model

[mnwhite]

The text in the rep agent example was copy-pasted from the medical shock model. It looks like some stuff got fixed, but I cleaned up the rest. Notebook should probably be run again by someone with the latest version of HARK installed locally. Only markdown cells changed on this commit. Fixes #964

Nothing added to changes log, as this just fixes example documentation.

[codecov-io]

Codecov Report

Merging #973 (db1815e) into master (1babcb9) will increase coverage by 0.41%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #973      +/-   ##
==========================================
+ Coverage   71.43%   71.84%   +0.41%     
==========================================
  Files          62       62              
  Lines        9145     9204      +59     
==========================================
+ Hits         6533     6613      +80     
+ Misses       2612     2591      -21     

Impacted Files	Coverage Δ
...nsumptionSaving/tests/test_IndShockConsumerType.py	100.00% <0.00%> (ø)
...tionSaving/tests/test_PerfForesightConsumerType.py	100.00% <0.00%> (ø)
HARK/ConsumptionSaving/ConsIndShockModel.py	86.61% <0.00%> (+3.22%)	⬆️

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