Update Emscripten-build-instructions.md to match PR in xeus-cpp https://github.com/compiler-research/xeus-cpp/pull/199

[mcbarton]

Description

Please include a summary of changes, motivation and context for this PR.

This updates the emscripten build instruction to match the PR in xeus-cpp compiler-research/xeus-cpp#199

Fixes # (issue)

Type of change

Please tick all options which are relevant.

• Bug fix
• New feature
• Requires documentation updates

Testing

Please describe the test(s) that you added and ran to verify your changes.

Checklist

• I have read the contribution guide recently

[anutosh491]

Looks good to me .

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.87%. Comparing base (6cc66be) to head (b4e0ff2).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #410   +/-   ##
=======================================
  Coverage   70.87%   70.87%           
=======================================
  Files           9        9           
  Lines        3533     3533           
=======================================
  Hits         2504     2504           
  Misses       1029     1029