Bug in `content-char`

@bhaible noticed that `content-char` says (in part):

```abnf
                  / %x0E-19        ; omit SP (%x20)
                  / %x21-2D        ; omit . (%x2E)
```

It should say:

```abnf
                  / %x0E-1F        ; omit SP (%x20)
                  / %x21-2D        ; omit . (%x2E)
```

Because hex works that way...