Fix source reference for monokai.js

What does this MR do and why?

This MR fixes an incorrect source reference made in monokai.js, introduced in 21fa36c7.

This helps to find the correct source of the theme.

References

This happened due to the instructions for adding Web IDE themes:

3. Copy over all the CSS variables from _dark.scss to _solarized_dark.scss and assign them your own values. Put them under the selector .ide.theme-solarized-dark.

MR acceptance checklist

• The MR was evaluated against the MR acceptance checklist.

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Chat

I had a larger project in mind as my first contribution, which was implementing Catppuccin Mocha as a theme, but then I looked into the files more and realized that this was beyond my current skills.

Closes #514241 (closed)