Fix marker-pdf version

Bump marker-pdf version to accommodate for the changes in surya-models. The changes break download.py and upgrading to 0.2.16 solves the issue. See changes [here](https://github.com/VikParuchuri/marker/pull/229/files#diff-07521f2e4d8e654778fb293b64ebae525c92042854c24421fa1b123f2555644fR6)