ruff_python_formatter: fix typo in code

When I renamed `line` to `trimmed`, I forgot one.

(Previously, I had rebound `line`, and I think that made
these sorts of bugs impossible.)

This regression is covered by new tests for Markdown supported
in a subsequent commit.

crates/ruff_python_formatter/src/expression/string/docstring.rs

@@ -1306,7 +1306,7 @@ fn indentation_trim(indent_len: TextSize, line: &str) -> &str {
             break;
         }
     }
-    line
+    trimmed
 }
 
 /// Returns the indentation of the given line and everything following it.