fix formatting error

Author: charlesdong1991

Diff for: pandas/core/strings.py

@@ -581,9 +581,9 @@ 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, " +
-                          f"not regex. The default will change in the future",
-                          FutureWarning)
+            warnings.warn("'%s' is interpreted as a literal in " % pat +
+                          "default, not regex. The default will change in the "
+                          "future", FutureWarning)
         f = lambda x: x.replace(pat, repl, n)
 
     return _na_map(f, arr)