Implementation of Modulo operator in documentation 
What database driver are you using this with, out of curiosity? The example works fine with Sqlite but I'm wondering if this is a result of psycopg2 doing its mogrifying client-side. This was with Postgre. Indeed, the issue was raised on l. 3144, `cursor.execute(sql, params or ())`.
## Problem

In the [documentation](https://docs.peewee-orm.com/en/latest/peewee/query_operators.html?#adding-user-defined-operators), it is said that the modulo operator can be implemented with
```python
from peewee import *
from peewee import Expression # the building block for expressions

def mod(lhs, rhs):
    return Expression(lhs, '%', rhs)
```

This gives a `IndexError: list index out of range`, due to the `%` causing string formatting issues. 

## Fix

Replacing `%` with `%%`. 
```python
def mod(lhs, rhs):
    return Expression(lhs, '%%', rhs)
```

That's not a big issue, but when we know nothing about Python 2 string formatting and SQL, it's a bit unsettling :) 