Fix typo and incorrect variable name in balance.c comment

[Copilot]

Final review of PR #248 identified a typo in src/sipnet/balance.c line 21.

Changes

• Fixed typo: "we ust plantWoodC" → "we use plantWoodC"
• Corrected variable name reference: "plantNSCWoodCDelta" → "plantWoodCStorageDelta"

The comment now accurately describes the nitrogen accounting logic where plantWoodC is used without the storage delta component.

// Note: this is the one place where we use plantWoodC by itself; it's the
// reason plantWoodCStorageDelta was created, so that we can ignore it here.

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[Alomir]

LGTM