Fix spelling mistakes

Found with
```bash
find -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. (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.)