update position correctly after reading fewer than length bytes

jvector-examples/src/main/java/io/github/jbellis/jvector/example/util/MMapReader.java

@@ -37,7 +37,7 @@ private void read(byte[] bytes, int offset, int count) {
         try {
             buffer.memory().getBytes(position, bytes, offset, count);
         } finally {
-            position += bytes.length;
+            position += count;
         }
     }