Clarify that static variables are not synonymous with global variables

[seven7763]

Summary

Fixes #4819.

Section 20.1 said "In Rust, global variables are called static variables", which reads as global == static. A static item is defined by its 'static lifetime, not by its scope: per the Reference, static items may be declared at module scope or inside a function. This rewords the paragraph to make that distinction while keeping the section's flow into Listing 20-10.

Changes

• src/ch20-01-unsafe-rust.md: reworded one paragraph (3 lines) in the "Accessing or Modifying a Mutable Static Variable" section.

Validation

• Checked the wording against the Rust Reference on static items (statics have program lifetime and may appear in any scope)
• dprint check src/ch20-01-unsafe-rust.md — the two reported regions (lines ~337-343, ~493+) also fail on unmodified main; they are pre-existing and outside this change's lines (~390-400).

Notes

Kept the change minimal — no restructuring of the section, just the incorrect equivalence.