docs(widget): fix typo in arrange method docstring

Author: TomJGooding

src/textual/widget.py

@@ -1267,7 +1267,7 @@ def render_str(self, text_content: str | Content) -> Content:
     def arrange(self, size: Size, optimal: bool = False) -> DockArrangeResult:
         """Arrange child widgets.
 
-        This method is best left along, unless you have a deep understanding of what it does.
+        This method is best left alone, unless you have a deep understanding of what it does.
 
         Args:
             size: Size of container.