Most people tend to apologize to their friends or families during normal conversations.
While that may reflect courteousness in normal opinion giving talks, it is should be avoided during public speaking.
Remember you have a message to give your audience; the audience also wants you to be informative, thoughtful and interesting, so why should you apologize.
Apologizing show doubts for your own beliefs and convictions. The audience will never notice anything you might have missed. So relax, focus your attention on your message, not your anxieties and rule of thumb- don’t apologize.