Back in the days of typewriters, depending on the age of the machine, letters would wear and become somewhat indistinct. An "i" could easily start to look like an "l".
I'm so old my first typewriter didn't even have a "1" (that's a numeral "one") -- you were expected to use the "l" (that's a lower-case "el."). It was a portable and I suppose leaving off that key saved weight. Maybe a whole half-ounce or so!
I just took it that Norville simply had GRAVITY foremost on his mind. I mean he was sitting in mid air.. so I think its not that he can't read so much as his mind was making the connection to the word FALL.. over and over. Then his mind ran it backwards in a kind of "over correct". That's how I always took the scene. Not that he couldn't read.