[pandas_panel.md] Use direct raw.githubusercontent.com URLs for the data-lectures datasets #580

#578 pointed the three dataset reads in pandas_panel.md at https://github.com/QuantEcon/data-lectures/raw/main/lectures/{realwage,countries,employ}.csv, a URL form that 302-redirects to raw.githubusercontent.com.

Direct raw URLs (https://raw.githubusercontent.com/QuantEcon/data-lectures/main/lectures/....csv) skip the redirect, are friendlier to strict HTTP clients and mirrored environments, and match the form these same lines used before #578 as well as the convention elsewhere in the lecture series.

All three current URLs do resolve (verified 2026-07-18: one redirect, then 200), so this is a minor robustness/consistency improvement rather than a breakage — and possibly a deliberate choice, in which case feel free to close.

Origin: flagged by Copilot review on the sync PRs QuantEcon/lecture-python-programming.zh-cn#72 and QuantEcon/lecture-python-programming.fa#136. The URLs mirror the English source, so any change belongs here and will propagate to all editions through translation sync.