configs/raspberrypizero2w_defconfig: fix typo in headers series option

The build for this defconfig fails with:

  Incorrect selection of kernel headers: expected 6.8.x, got 6.1.x

Found with the following command while trying to find boards that have
missing custom headers series:

  grep --null --recursive --files-without-match "BR2_TOOLCHAIN_EXTERNAL=y" ./configs \
    | xargs --null grep --null --files-with-matches "BR2_LINUX_KERNEL_CUSTOM_.*=y" \
    | xargs --null grep --null --files-without-match "BR2_KERNEL_HEADERS_[0-9]\+_[0-9]\+=y"\
    | xargs --null grep --files-without-match "BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_[0-9]\+_[0-9]\+=y"
  ./configs/mangopi_mq1rdw2_defconfig  # patch pending
  ./configs/raspberrypizero2w_defconfig
  ./configs/sipeed_licheepi_nano_defconfig  # patch pending

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/6712162631

Signed-off-by: Gero Schwäricke <gero.schwaericke@posteo.de>
[Romain: add gitlab-ci link]
Signed-off-by: Romain Naour <romain.naour@smile.fr>

configs/raspberrypizero2w_defconfig

@@ -3,7 +3,7 @@ BR2_cortex_a53=y
 BR2_ARM_FPU_NEON_VFPV4=y
 
 # Linux headers same as kernel, 6.1 series
-BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_6.1=y
+BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_6_1=y
 
 BR2_TOOLCHAIN_BUILDROOT_CXX=y