Fix spelling mistakes and typing errors

Found with

    find source -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.

See also ceylon/ceylon.language#512 and ceylon/ceylon.language@28e2b25.
Merge pull request #263 from lucaswerkmeister/master

Fix spelling mistakes and typing errors