Fix typo for system dark theme with entra id (#440)

Author: timja

src/main/java/io/jenkins/plugins/darktheme/DarkThemeSystemManagerFactory.java

@@ -21,7 +21,7 @@ public Theme getTheme() {
         return Theme.builder()
             .respectSystemAppearance()
             .withProperty("ace-editor", "theme-dark", ACE_EDITOR_THEME)
-            .withProperty("entra-id", "theme", ENTRA_ID_THEME)
+            .withProperty("entra-id", "theme-dark", ENTRA_ID_THEME)
             .withProperty("prism-api", "theme-dark", PRISM_THEME)
             .withProperty("bootstrap", "theme-dark", BOOTSTRAP_THEME)
             .build();