a typo

[wojdyr]

Sorry to bother you with such a trivial matter.
I like to use codespell to check my code, and it flags one typo in JS generated by emscripten:

mtz.js:8: nonexistant ==> nonexistent

It originates from this line:

emscripten/src/embind/embind.js

Line 153 in e0c3ce6

throwInternalError('Replacing nonexistant public symbol');

[sbc100]

Feel free to send a PR to fix this.