Correct the "Why Open-Source" URL

[trunghaiy]

Fix the broken link

Describe Your Changes

Use "https://www.jan.ai/handbook/why/betting-on-open-source" instead of "https://www.jan.ai/handbook/betting-on-open-source"

Self Checklist

• [x] Added relevant comments, esp in complex areas
• [x] Updated docs (for bug fixes / features)