docs(guide/Expressions): Redirection to MDN

[franciscovelez]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
docs update

What is the current behavior? (You can also link to an open issue here)

What is the new behavior (if this is a feature change)?

Does this PR introduce a breaking change?

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md#commit-message-format
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:

It is better to link to the English version of MDN rather than German version

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[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[franciscovelez]

I signed it!

[Narretz]

I agree. Or maybe simply leave out the local modifier? That works too

[franciscovelez]

I think that is better use the local modifier to avoid the 301 redirection from "no local modifier" to "en-US" page

[Narretz]

Ok, that makes sense. Regarding the CLA, did you sign it with the same email with which you authored the commit? Otherwise it won't detect it.

[franciscovelez]

OK, I have signed the CLA with my other email

[googlebot]

CLAs look good, thanks!

[googlebot]

CLAs look good, thanks!