Broken link to "ICU message syntax"

[zzzzBov]

• [x] I am on the latest ember-intl version
• [x] I have searched the issues of this repo and believe that this is not a duplicate

Environment

n/a

Steps to Reproduce

The "ICU message syntax" link in the docs (src) links to https://formatjs.io/docs/icu-syntax which doesn't exist.

I'd update the link in a PR, but I'm not certain where it should point to.

Here are a couple candidates I found while searching for the ICU message syntax:

• https://formatjs.io/docs/intl-messageformat/#message-syntax
• http://userguide.icu-project.org/formatparse/messages#TOC-MessageFormat

[jasonmit]

Thanks! Will appear in the docs once I version a patch in a few minutes.

[jasonmit]

Fixed v.5.4.2 (deploying now)