Fix spelling mistakes

Found with

    find -name '*.ceylon' -exec grep '^\(    \)*\( \|"\)[^ ]' {} + > tmp

to find text that's most likely documentation (starts with 4n + 1
spaces), and then

    hunspell tmp

to spellcheck the text. (Most of the unknown words were still not actual
typos (references to classes and functions), but this is still better
than just spellchecking the entire code, where there would be even more
such false positives.)