fix(progressbar): add missing i18n id specifier

[markpeterfejes]

The id was picked up as description by the angular cli, which resulted in a random id:

<trans-unit id="04d611d19c117c60c9e14d0a04399a027184bc77" datatype="html">
    <source><x id="INTERPOLATION" equiv-text="{{getPercentValue()}}"/>%</source>
    <context-group purpose="location">
      <context context-type="sourcefile">../node_modules/@ng-bootstrap/ng-bootstrap/progressbar/progressbar.d.ts</context>
      <context context-type="linenumber">6</context>
    </context-group>
    <note priority="1" from="description">ngb.progressbar.value</note>
</trans-unit>

[maxokorokov]

Thanks!