Fix typo reported at https://sourceforge.net/p/giflib/bugs/139
Typos: "0 it 255" -> "0 to 255", "let's us know" -> "lets us know"
A few more minor fixes (it's -> its, sting -> string)
Fix minor typos/spelling errors
Minor function/parameter name fixups
GIF89a.html - Replace Unicode chars with HTML entities
Fix typo reported at https://sourceforge.net/p/giflib/bugs/139
Typos: "0 it 255" -> "0 to 255", "let's us know" -> "lets us know"