Improve list builder usage example in docs

The example could leave the impression that it is important to append some
dummy element (which value is irrelevant) to values builder when
constructing null list entry.

arrow-array/src/builder/generic_list_builder.rs

@@ -49,7 +49,6 @@ use std::sync::Arc;
 /// builder.append(true);
 ///
 /// // Null
-/// builder.values().append_value("?"); // irrelevant
 /// builder.append(false);
 ///
 /// // [D]
@@ -70,15 +69,14 @@ use std::sync::Arc;
 ///   array.values().as_ref(),
 ///   &StringArray::from(vec![
 ///     Some("A"), Some("B"), Some("C"),
-///     Some("?"), Some("D"), None,
-///     Some("F")
+///     Some("D"), None, Some("F")
 ///   ])
 /// );
 ///
 /// // Offsets are indexes into the values array
 /// assert_eq!(
 ///   array.value_offsets(),
-///   &[0, 3, 3, 4, 5, 7]
+///   &[0, 3, 3, 3, 4, 6]
 /// );
 /// ```
 ///