Ensure that when an overlay is added that the class `cm-overlay` is
used to denote a `span` instead of `cm-cm-overlay` (with the `cm-`
prefix repeated twice).

* E.g. before fix:
```
<span class="cm-cm-overlay cm-spell-error">zpelling</span>
```

* And after fix:
```
<span class="cm-overlay cm-spell-error">zpelling</span>
```