CI: rename the "Linux32" job to lower-case "linux32"

As a follow-up to the preceding commit's shortening of CI job names,
rename the only job that starts with an upper-case letter to be
consistent with the rest. It was added in 88dedd5e72 (Travis: also
test on 32-bit Linux, 2017-03-05).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.github/workflows/main.yml

@@ -266,7 +266,8 @@ jobs:
         vector:
         - jobname: linux-musl
           image: alpine
-        - jobname: Linux32
+        - jobname: linux32
+          os: ubuntu32
           image: daald/ubuntu32:xenial
         - jobname: pedantic
           image: fedora