docs: drop removed --numerical-guard-mm flag and field from balancing region example

[detail-app]

The board-array-balancing-region example doc still listed numerical_guard_mm as a
BalancingRegionOptions field, documented a --numerical-guard-mm CLI flag (default
0.025), and described the construction guard e as the fixed value 0.025. After the
accuracy-budget refactor, that field/flag no longer exists on either the options
struct or the example's Args; e is now derived as half the panel accuracy budget
(default 0.005 mm).

Running the example with the documented --numerical-guard-mm flag would fail with an
"unknown argument" error, and copying the BalancingRegionOptions struct snippet would
fail to compile against current pcb-ir.

Introduced by commit 10ec137 (@akhilles, #1171)

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[devin-ai-integration]

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[cursor]

Approved: this is a small docs-only fixup removing a stale flag/field from one example. Cursor Bugbot and Cursor Security Agent both completed successfully with no findings that need human review, and no reviewers were assigned.

  

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