Fix calculation for buffer duration in basic-tutorial-8

We need to scale the number of samples, not the buffer size.

tutorials/src/bin/basic-tutorial-8.rs

@@ -164,7 +164,7 @@ fn main() {
                         .mul_div_floor(data.num_samples, SAMPLE_RATE as u64)
                         .expect("u64 overflow");
                     let duration = gst::SECOND
-                        .mul_div_floor(CHUNK_SIZE as u64, SAMPLE_RATE as u64)
+                        .mul_div_floor(num_samples as u64, SAMPLE_RATE as u64)
                         .expect("u64 overflow");
 
                     {