Outdated `gpuOffset` references in Python model load config docs

[ncoghlan]

The config API used to include a gpuOffload config field with a floating point value.

This was changed to a gpu settings dict some time ago, but the Python SDK docs are still describing the previous format (resulting in questions like lmstudio-ai/lmstudio-python#88).

(The Typescript SDK docs were updated back in #41)

[ncoghlan]

Related lmstudio-js issue: lmstudio-ai/lmstudio-js#67