phosphor-logging: fix yaml option

The meson option in the repository is 'yamldir' and not 'yaml_dir'.
Fix this since later versions of Meson complain about an unknown
option being passed.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I4241745e6cafe310152e8d8235db012af447a167
diff --git a/scripts/build-unit-test-docker b/scripts/build-unit-test-docker
index 1a744e8..6962658 100755
--- a/scripts/build-unit-test-docker
+++ b/scripts/build-unit-test-docker
@@ -239,7 +239,7 @@
         ],
         build_type="meson",
         config_flags=[
-            f"-Dyaml_dir={prefix}/share/phosphor-dbus-yaml/yaml",
+            f"-Dyamldir={prefix}/share/phosphor-dbus-yaml/yaml",
         ],
     ),
     "openbmc/phosphor-objmgr": PackageDef(