Update src/icalendar/cal.py

Co-authored-by: Steve Piercy <web@stevepiercy.com>

src/icalendar/cal.py

@@ -568,7 +568,7 @@ def __eq__(self, other):
     """)
 
     def is_thunderbird(self) -> bool:
-        """Whether this component has attributes that indicate that Mozilla Thunderbird createsd it."""
+        """Whether this component has attributes that indicate that Mozilla Thunderbird created it."""
         return any(attr.startswith("X-MOZ-") for attr in self.keys())