Fixed Trainer default labels for QA Model
PR https://github.com/huggingface/transformers/pull/7191 added default labels for models. However, for QA Models it sets the default labels to ["start_positions, end_positions"] instead of ["start_positions", "end_positions"], which was the intention based on the PR description. This PR is a simple fix to this.

Fixes #8390 

@sgugger 