Make nav-title 100% width in mobile viewport

[marinaaisa]

Bug/issue #137436968, if applicable:

Summary

This is a small change to make the nav-title to be 100% width in mobile viewport instead of 90%.
There was no reason for it to be 90% instead.

Dependencies

NA

Testing

Steps:

1. Go to any documentation or tutorial page
2. Assert that the nav title looks correct in mobile viewport

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests
• Ran npm test, and it succeeded
• Updated documentation if necessary

[mportiz08]

This works better in the common case, but we have overflow issues when the title here is really long:

Is this something we could easily address in the same PR possibly?