fix DeprecationWarning: escape sequences for normalize_string()

[f-hafner]

DeprecationWarning for normalize_string():

src/dataprep/helpers/functions.py:41
  /home/flavio/projects/mag_sample/src/dataprep/helpers/functions.py:41: DeprecationWarning: invalid escape sequence \w
    s = s.replace("[^\w\d]", " ", regex = True)

src/dataprep/helpers/functions.py:42
  /home/flavio/projects/mag_sample/src/dataprep/helpers/functions.py:42: DeprecationWarning: invalid escape sequence \s
    s = s.replace("\s+", " ", regex = True)

these should be replaced with r"[^\w\d]" and with r"\s+" respectively.

see also here