Avoid deprecation warning

[FlorianPfaff]

Hi,

I was just tired of getting the filterpy/common/helpers.py:364: DeprecationWarning: invalid escape sequence '\S' warning. I know that since a raw string is used, it should be no issue, but it keeps coming up in my logs. Hence, I changed it. I think this should be fine since it says that the \ is not an escape character for Python but part of the (LaTeX) text.