[review] fix docstring

sentry-core/src/performance.rs

@@ -187,7 +187,7 @@ impl TransactionContext {
 
     /// Update the custom context of this Transaction.
     ///
-    /// For simply adding a key, use the `set_custom_key` method.
+    /// For simply adding a key, use the `custom_insert` method.
     pub fn custom_mut(&mut self) -> &mut Option<CustomTransactionContext> {
         &mut self.custom
     }