docs: fix camel_to_snake docstring

Spotted in passing and decided just to fix it after wondering if it was
even worth a pull request.

Author: TomJGooding

src/textual/case.py

@@ -11,7 +11,7 @@ def camel_to_snake(
         name: A symbol name, such as a class name.
 
     Returns:
-        Name in camel case.
+        Name in snake case.
     """
 
     def repl(match: Match[str]) -> str: