Correct 'milli' in core/form.js

Problem/Motivation

cspell identifies the use of 'milli' in this sentence as a misspelling.

   *   (due to limitations of jQuery.serialize()). That is deemed to be
   *   acceptable, because if the user forgot to attach a file, then the size of
   *   HTTP payload will most likely be small enough to be fully passed to the
   *   server endpoint within {milli}seconds. If a user mistakenly attached a
   *   wrong file and is technically versed enough to cancel the form submission
   *   (and HTTP payload) in order to attach a different file, then that
   *   edge-case is not supported here.

The use of milli first appeared in the patch at #1705618-21: Double click prevention on form submission and was in commit 502e953.

Steps to reproduce

Proposed resolution

In #3397490: Fix more words that are only misspelled in comments xjm suggested "the author meant to say something like"

   *   server endpoint within seconds, or even milliseconds. If a user
   *   mistakenly attached a wrong file and is technically versed enough to
   *   cancel the form submission (and HTTP payload) in order to attach a
   *   different file, then that edge-case is not supported here.

Remaining tasks

Determine the correct change so milli is not detected as a spelling error and the comment is correct.

User interface changes

API changes

Data model changes

Release notes snippet