fix: typo in MultiTimeUnit declaration

[jkillian]

Fixes a typo in MULTI_TIMEUNIT declaration where a value was specified as "weeksdayhours" instead of "weekdayhours".

I'm not sure if this is a breaking change - if people are validating a Vega-Lite schema and have weeksdayhours values in it, the validation will now fail. However, it seems like the old value was a clear mistake as every other value uses a singular week and not a plural weeks, so maybe it makes sense to fix this anyways if it falls into the "bug" category of change? Up to you all of course, feel free to do with this PR whatever is best!

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[domoritz]

This looks like a bug we should fix. Thanks for the pull request.