Remove extra whitespace in the warning message

Author: roberthdevries

pandas/io/excel/_base.py

@@ -855,8 +855,8 @@ def __init__(self, path_or_buffer, engine=None):
 
         if engine == "xlrd":
             warnings.warn(
-                'The Excel reader engine will default to "openpyxl" in the future. \
-                 Specify engine="openpyxl" to suppress this warning.',
+                'The Excel reader engine will default to "openpyxl" in the future. '
+                'Specify engine="openpyxl" to suppress this warning.',
                 FutureWarning,
             )
         if engine not in self._engines: