docs(guide/Directives): Use data-ng-model

[eungjun-yi]

Use data-ng-model instead of data-ng:model which is accepted for legacy reason.
The next "Normalization" section is saying:

Best Practice: Prefer using the dash-delimited format (e.g. ng-bind for
ngBind). If you want to use an HTML validating tool, you can instead use the
data-prefixed version (e.g. data-ng-bind for ngBind). The other forms
shown above are accepted for legacy reasons but we advise you to avoid
them.

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[eungjun-yi]

I signed it!

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