One day Ruky’s “Cor! Cor!” had sounded oftener than usual. His rosy face had bent saucily to kiss Cora’s upturned forehead, as she raised her arms to lift him from the tree; but the sparkle in his dark eyes had seemed to kindle so much mischief in him that his sister’s patience became fairly exhausted.

he used semi colon incorrectly am i right?

No, the semi-colon is used correctly in the given sentence. It is used to separate two independent clauses that are closely related in meaning.