Rename api.HTMLImageElement.lowSrc -> lowsrc

[queengooborg]

This PR corrects capitalization for the HTMLImageElement API's lowsrc feature.

[foolip]

This looks good, but I'm not sure how to do the rename dance with https://wiki.developer.mozilla.org/en-US/docs/Web/API/HTMLImageElement/lowSrc in a way that doesn't break the compat table there for longer than necessary. @ddbeck can you advise?

[foolip]

I have done all the renaming in https://wiki.developer.mozilla.org/en-US/docs/Web/API/HTMLImageElement and https://wiki.developer.mozilla.org/en-US/docs/Web/API/HTMLImageElement/lowsrc other than the name in the BCD reference.

[ddbeck]

@foolip Since I'll see this when I do the release notes anyway, I'll update the compat macro on the page at that time. Is that satisfactory? Do we need anything else to merge this?

[foolip]

@ddbeck that sounds good, getting the timing right is the only thing I'm concerned about, since the worst case is that the compat table is broken for a week, and the best case is that nobody notices at all :)

I'll go ahead and merge this then.