Issue title: Documentation: ellipsis or dot dot dot?
 Issue body: At: https://shopify.github.io/liquid/filters/truncate/

Documentation states:

> If the specified number of characters is less than the length of the string, an ellipsis (…) is appended to the string and is included in the character count.

and gives a contradictory example:

> Ground control to...

This is a contradiction because the former is an ellipsis and the latter is three periods.

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PR coming
 Comments: 
 Comment 0: Correction. Sorry PR not coming.

I thought this was a problem with the documentation at https://github.com/Shopify/liquid/blob/gh-pages/_filters/truncate.md

But now I see this a problem with Liquid failing to produce the behavior required from the documentation.

To be clear: Liquid should output an actual ellipsis rather than three dots.
 Comment 1: The behaviour in the docs seems better, but I think the docs are incorrect here, probably due to automatic conversion by Markdown/Jekyll.
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