fix formatting error

Author: charlesdong1991

pandas/core/strings.py

@@ -581,7 +581,7 @@ def str_replace(arr, pat, repl, n=-1, case=None, flags=0, regex=None):
         # if regex is default None, and a single special character is given
         # in pat, still take it as a literal, and raise the Future warning
         if regex is None and len(pat) == 1 and re.findall(r"\W", pat):
-            warnings.warn(f"{pat} is interpreted as a literal in default, "
+            warnings.warn(f"{pat} is interpreted as a literal in default, " +
                           "not regex. The default will change in the future",
                           FutureWarning)
         f = lambda x: x.replace(pat, repl, n)