Update DocModel.php

[LDAV]

${} for string interpolation inside double quotes in PHP is deprecated

Issue #, if available:

Description of changes:

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[stobrien89]

Hi @LDAV,

Thanks for the contribution. In this particular case, we're trying to access variables within the doc model that are prefixed with $, so we're not actually using it for interpolation (hence the escape char). I'd prefer to keep the existing code. Let us know if you have any questions.