Fix issue found by latest version of flake8

Author: sloria

dev-requirements.txt

@@ -2,4 +2,4 @@ nose>=1.3.0
 tox>=2.6.0
 invoke>=0.15.0
 mock==2.0.0
-flake8==3.5.0
+flake8==3.7.4

textblob/blob.py

@@ -575,7 +575,7 @@ def correct(self):
         :rtype: :class:`BaseBlob <BaseBlob>`
         """
         # regex matches: word or punctuation or whitespace
-        tokens = nltk.tokenize.regexp_tokenize(self.raw, "\w+|[^\w\s]|\s")
+        tokens = nltk.tokenize.regexp_tokenize(self.raw, r"\w+|[^\w\s]|\s")
         corrected = (Word(w).correct() for w in tokens)
         ret = ''.join(corrected)
         return self.__class__(ret)