chore(input): fix selector on mdInput

[kara]

Updates the selector for mdInput within md-input-container to match other attribute directives.

[francisoud]

You should add a "breaking change" mention the changelog to tell people that they need to update from md-input to mdInput when moving from 2.0.0-beta.1 to the next version... otherwise they will discover it the 'hard way' ;)
I don't see any documentation change in 93b0bc9

[mmalerba]

@francisoud I think the README was updated as part of this pr, but you're right we should call this out in the changelog for the next release, thank you for the reminder.

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