Correct error in tuple example in ruff formatter docs (#7822)

## Summary

The fourth element should be "d" instead of "c" in the tuple example in
the ruff formatter docs.

## Test Plan

N/A

crates/ruff_python_formatter/README.md

@@ -428,7 +428,7 @@ In particular, Ruff will always insert parentheses around tuples that expand ove
     (a, b),
     (
         c,
-        c,
+        d,
     ),
 )
 ```