Fix "Preemtive" mode name

[dgw]

Saw Xterminator's spotlight of this mod today and thought I could make a small, but useful, contribution.

• Added migration script to convert old saves using the original mode name "Preemtive" to the new name "Preemptive"
  • I tested by adding the released ZIP of 18.1.6 to a copy of one of my test maps, checking its setting value ("Preemtive"), then quitting Factorio and replacing the ZIP with the unzipped folder containing my changes. No errors were thrown, and the stored setting was updated to "Preemptive" as expected upon loading the test map again.
• Fixed whitespace around top of README, especially the animated demo
• Tweaked a couple other minor spelling/grammar errors

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If you're so inclined, and you like this PR, having it counted toward my Hacktoberfest statistics would be great! But I still would have opened it regardless, and I completely understand if you don't want to add any label/topic to this PR/repo just for me. 😸

[muppet9010]

changes merged manually in to: 89df3525a6b013a6e3fc35c08e0348d3b684bdf5 as I don't like GitHub's PR merge view and use a standalone GUI GitHub manager.

[dgw]

Kind of curious what GUI that is, since it appears not to preserve any original authorship info. That's unusual for Git tools.

Really appreciate the hacktoberfest label, thanks!

[muppet9010]

I use GMaster. It's an in-development GUi by the company that makes PlasticSCM, an alternative source control tool to GIT. It may have PR management, but I haven't found it. So the few PRs I have, I manually download the changes, then apply and tweak them to fit my style, then test and commit. Hence the loss of any source ownership to the PR.