Fix erratic learning-rate example

[ziyecc-tech]

Summary:

• Correct the erratic learning curves example so the diff lowers the learning rate from 1e-4 to 1e-5.
• Apply the same numeric fix to the English, Korean, and Burmese chapter files that contain the same example.

Verification:

• git diff --check
• rg -n --multiline "-learning_rate=1e-5,\n\s*+learning_rate=1e-4" chapters/en/chapter3/5.mdx chapters/my/chapter3/5.mdx chapters/ko/chapter3/5.mdx

Fixes #1240