Fixes broken Handlebar documentation links

[jamiesmith]

Summary

Just updated the URL for handlebar docs, page moved:

• https://handlebarsjs.com/expressions.html -> https://handlebarsjs.com/guide/expressions.html
• http://handlebarsjs.com/expressions.html -> https://handlebarsjs.com/guide/expressions.html

Checklist

Use strikethroughs to remove checklist items you don't feel are applicable to this PR.

• This was checked for cross-browser compatibility, including a check against IE11
• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• This was checked for keyboard-only and screenreader accessibility

For maintainers

• This was checked for breaking API changes and was labeled appropriately
• This includes a feature addition or change that requires a release note and was labeled appropriately

[jamiesmith]

Note that I don't know how to backport this, just fixing the URL

[tylersmalley]

LGTM

$ curl -I https://handlebarsjs.com/guide/expressions.html                 ✔ HTTP/2 200
server: GitHub.com
content-type: text/html; charset=utf-8
last-modified: Mon, 13 Jan 2020 20:59:28 GMT
etag: "5e1cda30-19b34"
access-control-allow-origin: *
expires: Fri, 24 Jan 2020 17:50:36 GMT
cache-control: max-age=600
x-proxy-cache: MISS
x-github-request-id: 9376:605F:3DF852:45DC9D:5E2B2C12
accept-ranges: bytes
date: Fri, 24 Jan 2020 17:40:42 GMT
via: 1.1 varnish
age: 6
x-served-by: cache-sea4423-SEA
x-cache: HIT
x-cache-hits: 1
x-timer: S1579887642.433167,VS0,VE1
vary: Accept-Encoding
x-fastly-request-id: 9db809b7a1d9c10ff3dd7ca3325e2b102e1362f8
content-length: 105268

[poffdeluxe]

Hmmm I guess I can let this one slide

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: d935c5e

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[bhavyarm]

@jamiesmith I came across this broken link while testing 7.6.0 bc2 on this pr: #54194. Can you please backport this fix to 7.6? The next bc will get it. Thanks!

[jamiesmith]

@bhavyarm I ran the backport script and it shows that it was applied, above. Is there something else?

[crob611]

@jamiesmith It was backported to 7.x, but the 7.6 branch had already been split off. You should be able to run the backport script again, but select the 7.6 branch instead of 7.x