Fix bugs of the documentation of Pyomo.DoE

[jialuw96]

Summary/Motivation:

This fixes the problem that in the documentation, step 4: enumeration and step 5: gradient-based optimization, show the same code.

Changes proposed in this PR:

• in doe.rst step 4 enumeration, change the code included in this part to reactor_grid_search.py
• in doe.rst step 5 optimization, change the code included in this part to reactor_optimize_doe.py

Legal Acknowledgement

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1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[adowling2]

@blnicho @jsiirola @mrmundt Shall fix to the documentation.

[jsiirola]

The failing Jenkins tests are unrelated to this PR. This is safe to merge

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (809a58a) 88.09% compared to head (5dc5cd5) 88.09%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3070   +/-   ##
=======================================
  Coverage   88.09%   88.09%           
=======================================
  Files         774      774           
  Lines       91537    91537           
=======================================
+ Hits        80642    80643    +1     
+ Misses      10895    10894    -1     

Flag	Coverage Δ
linux	85.45% <ø> (ø)
osx	75.47% <ø> (ø)
other	85.63% <ø> (ø)
win	82.79% <ø> (ø)

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