Fixed examples in working_models.html to Run

[MrGorbunov]

Fixes # .

Code examples in docs now run verbatim

Summary/Motivation:

Two examples from the docs did not run when copy pasted

Changes proposed in this PR:

In working_models.rst:

• 'summation' was used instead of 'pyo.summation'
• An example script instantiated an optimizer twice. First with glpk and then with cplex. Removed the cplex.

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[jsiirola]

This is good, but why didn't the doc tests catch the error?

[blnicho]

This is good, but why didn't the doc tests catch the error?

I don't think we're running the test_examples.py script found here: https://github.com/Pyomo/pyomo/blob/main/doc/OnlineDocs/tests/test_examples.py. See issue #1950

[codecov]

Codecov Report

Merging #2502 (1fdbbf6) into main (a4f58ab) will decrease coverage by 0.00%.
The diff coverage is n/a.

❗ Current head 1fdbbf6 differs from pull request most recent head 23bcabc. Consider uploading reports for the commit 23bcabc to get more accurate results

@@            Coverage Diff             @@
##             main    #2502      +/-   ##
==========================================
- Coverage   86.00%   86.00%   -0.01%     
==========================================
  Files         715      714       -1     
  Lines       80116    80107       -9     
==========================================
- Hits        68907    68898       -9     
  Misses      11209    11209              

Flag	Coverage Δ
linux	83.43% <ø> (-0.01%)	⬇️
osx	73.75% <ø> (-0.01%)	⬇️
other	83.62% <ø> (-0.01%)	⬇️
win	80.59% <ø> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pyomo/contrib/appsi/solvers/gurobi.py	84.76% <0.00%> (-0.04%)	⬇️
pyomo/common/shutdown.py

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