fix twitter bootstrap link

[gbezyuk]

url of twitter bootstrap was outdated, fixed now

[mary-poppins]

Thanks for the PR!

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Thanks again for your help!

[IgorMinar]

I'm sorry, but I wasn't able to verify your CLA signature. CLA signature is required for any code contributions to AngularJS.

Please sign our CLA and ensure that the CLA signature email address and the email address in this PR's commits match.

If you signed the CLA as a corporation, please let me know the company's name.

Thanks a bunch!

PS: If you signed the CLA in the past then most likely the email addresses don't match. Please sign the CLA again or update the email address in the commit of this PR.
PS2: If you are a Googler, please sign the CLA as well to simplify the CLA verification process.

[gbezyuk]

CLA is signed twice already ) hope three times will be enough :-D

[caitp]

@gbezyuk, be sure you've signed the CLA with an email address matching the email address you've authored your patch with (git config --get user.email)

[petebacondarwin]

You can see what email address to use here: https://github.com/angular/angular.js/pull/5290.patch