fix SMT-LIB2 nor

[kroening]

This fixes the implementation of nor in the SMT-LIB2 backend, and the comment that describes the semantics of nor_exprt.

• [X ] Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.00%. Comparing base (af0b511) to head (d30e0af).
⚠️ Report is 4 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/solvers/smt2/smt2_conv.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8832   +/-   ##
========================================
  Coverage    80.00%   80.00%           
========================================
  Files         1700     1700           
  Lines       188248   188248           
  Branches        73       73           
========================================
  Hits        150613   150613           
  Misses       37635    37635           

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