Fix misspellings of 'compatible'/'compatibility' in OpenAIService

[5jdcdr57gf-stack]

Summary

Fixes four spelling errors of compatible/compatibility in marker/services/openai.py. One of them is in a user-facing config field description (openai_image_format), so it shows up in config --help output; the other three are in the process_images docstring.

Line	Before	After
30	...for better **compatability**	...for better **compatibility**
36	**openAI-compatabile** multimodal model.	**OpenAI-compatible** multimodal model.
40	...use "png" for better **compatability**.	...use "png" for better **compatibility**.
43	A list of **OpenAI-compatbile** multimodal messages...	A list of **OpenAI-compatible** multimodal messages...

Notes

• Documentation/string-only change — no behavior change.
• Verified the file still parses (python -c "import ast; ast.parse(...)").

🤖 Generated with Claude Code

[github-actions]

CLA Assistant Lite bot:
Thank you for your submission, we really appreciate it. Like many open-source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution. You can sign the CLA by just posting a Pull Request Comment same as the below format.

---

I have read the CLA Document and I hereby sign the CLA

---

_{You can retrigger this bot by commenting recheck in this Pull Request}