Audience: Anyone or a group of people watching something.
Spectators: Anyone watching but with a more focused feel to it. For example, the word "spectacle" means: "a visually striking performance or display." So think of spectators as being more visually engaged with what they are viewing.
Crowd: A crowd can be both above, but it must be A LOT of people. Audience and spectators can be small groups (or in some cases even one person, as in "an audience of one") but a crowd is when lots and lots of people gather in one place.
In your case, spectators is a good choice because they are talking about the first television program ever, which must of been a pretty "wow!" moment for those folks viewing it.
I hope this helps! Good luck!