According to the introduction to the book, what was the change to the name of the manuscript that Jim Burden gave to his friend? What is the significance of the change?

"I suppose it hasn't any form. It hasn't any title, either." He went into the next room, sat down at my desk and wrote on the pinkish face of the portfolio the word, "Antonia." He frowned at this a moment, then prefixed another word, making it "My Antonia." That seemed to satisfy him."

This reflects what Antonia meant to Jim, what the story is about.