* 'permission' instead of 'permissions'
* Also added a more user friendly error message when Attribute.string_to_permissions fails
```
TypeError: Attribute::permissions error:
Expected a string containing any of the keys, seperated by commas: READABLE,WRITEABLE,READ_REQUIRES_ENCRYPTION,WRITE_REQUIRES_ENCRYPTION,READ_REQUIRES_AUTHENTICATION,WRITE_REQUIRES_AUTHENTICATION,READ_REQUIRES_AUTHORIZATION,WRITE_REQUIRES_AUTHORIZATION
Got: 1
```
```
Exception: Error parsing Device Config's GATT Services. The key 'permission' must be renamed to 'permissions'
```
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