Drop smallpt notebook reference from Emscripten build instructions

[mcbarton]

Now that the smallpt example notebook has been dropped from xeus-cpp, references to it need to be dropped from CppInterOps Emscripten ci and documentation.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.04%. Comparing base (63fe46d) to head (0490d37).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #762   +/-   ##
=======================================
  Coverage   79.04%   79.04%           
=======================================
  Files           9        9           
  Lines        3880     3880           
=======================================
  Hits         3067     3067           
  Misses        813      813           

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[mcbarton]

pinging @vgvassilev @anutosh491 @vgvassilev to approve this PR, so we can get the Emscripten ci passing again. Given the smallpt notebook doesn't exist anymore, the ci will fail until this is in.