fix: semantic html for formula cannot be rendered as expect

[mxtaooo]

when apply semantic html for formula as content of editor, just print formula text.

Steps for Reproduction

1. create html with <div id="editor"></div> and editor creation related code, then load the page
2. enter a formula in editor, for exampe: x^2+y^2=1
3. take html string from getSemanticHTML(), then paste into html file, as the content of div#editor, then refresh browser

Expected behavior:

the formula should be pretty rendered

Actual behavior:

just print the text what we entered

Platforms:

Firefox

Version:

Quill 2.0.2