。
She made up her mind to go.
Literal
She [topic-は] go-decision [object-を] did.
決心をする ('to make up one's mind, to resolve') pairs the noun 決心 with する. The verb 行く directly modifies 決心 attributively — '[a] go decision' = 'decision to go.' This is a clean noun-modifier pattern: a plain-form verb (行く) directly modifies a noun (決心), no relative-pronoun overhead. Compare with the related 決意する ('to be resolved') and the casual ~ことにする ('to decide on doing'). 決心 lands somewhere in the middle of resolve-related vocabulary — more than a passing intent, less than a vow.