Fix spelling mistakes and typing errors

Found with
```bash
find source -name '*.ceylon' -exec grep '^\(    \)*\( \|"\)[^ ]' {} + > tmp
```
to find text that's most likely documentation (starts with 4n + 1 spaces), and then
```bash
hunspell tmp
```
to spellcheck the text.

See also ceylon/ceylon.language#512 and ceylon/ceylon.language@28e2b25.