Fix code error in examples for performance_tuning/regular_expressions.html

in code examples, 

```python
    # Soundex requirements:
    # source string must be at least 1 character
    # and must consist entirely of letters
    if (not source) and (not source.isalpha()):
        return "0000"
```

should be:

```python
    # Soundex requirements:
    # source string must be at least 1 character
    # and must consist entirely of letters
    if (not source) or (not source.isalpha()):
        return "0000"
```

Note: the files provided in diveintopython-examples-5.4.zip don't have the error