Fix formatting

Author: jsignell

xarray/util/deprecation_helpers.py

@@ -199,7 +199,7 @@ def warning_message(self, message: str, recommend_set_options: bool = True) -> s
 
         return (
             f"In a future version of xarray the default value for {self._name} will "
-            + f"change from {self._name}={self._old!r} to {self._name}={self._new!r}. "
+            f"change from {self._name}={self._old!r} to {self._name}={self._new!r}. "
             + message
             + recommendation
         )