Make documentation more clear (CTRL modifier)

In [wiremix.toml](https://github.com/tsowell/wiremix/blob/main/wiremix.toml#L76) it says the following:

```
#  { key = "End", modifiers = "CTRL | ALT", action = "Exit" },
```
Which is an invalid keysym.
It should say:
```
#  { key = "End", modifiers = "CONTROL | ALT", action = "Exit" },
```