DOC Update JS MIME type, remove type in <script>, fix few typos

[lerni]

Description

Update JS MIME type, remove type in <script> silverstripe/silverstripe-framework#11446, fix few typos

Issues

• remove application/javascript as this is not required with html5 silverstripe-framework#8710

Pull request checklist

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[GuySartorelli]

Thanks for this. Some suggested changes to wording below.

[GuySartorelli]

Should have asked for this last time, but can you please also prefix the commit message with DOC as per https://docs.silverstripe.org/en/5/contributing/code/#commit-messages

[GuySartorelli]

Looks good, thanks!