Update index.html

[mattbrundage]

A more succinct way of writing the conditional comment -- still recognized by IE 5-9.

IE10+ doesn't support conditional comments, however, IE10 would still benefit from the "outdated browser" message, as we recently dropped support.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[coliff]

Agreed this is a good change to save a couple of bytes... the way it works though is exactly the same since IE10 ignores conditional statements.
REF: https://docs.microsoft.com/en-us/previous-versions/windows/internet-explorer/ie-developer/compatibility/ms537512(v=vs.85)

[coliff]

please also make this change in the 'dist' dir.

[mattbrundage]

@coliff , I've made the change you requested.

[roblarsen]

🚢