[VariableBlockVector] Fix typo in comment

Status Job ID Name Coverage
  Test
failed #71854
duneci
debian:10 clang-7-libcpp-17

00:02:27

passed #71853
duneci
debian:10 gcc-8-noassert-17

00:04:02

passed #71856
duneci
debian:9 clang-3.8-14

00:02:15

passed #71855
duneci
debian:9 gcc-6-14

00:02:08

passed #71857
duneci
ubuntu:16.04 gcc-5-14

00:02:08

passed #71858
duneci
ubuntu:18.04 clang-6-17

00:03:01

 
Name Stage Failure
failed
debian:10 clang-7-libcpp-17 Test
Errors while running CTest
======================================================================
Name: dotproducttest
FullName: ./dune/istl/test/dotproducttest
Status: FAILED

--- Failed to build dune-istl ---
Terminating dunecontrol due to previous errors!
ERROR: Job failed: exit code 1