Added missing field mainOrganization

oparl-server/src/main/java/de/twomartens/oparlservice/entity/Body.java

@@ -132,4 +132,7 @@ public class Body {
 
     @Schema(description = "classification of the body")
     private final String classification;
+
+    @Schema(description = "URL to the main organization of this body", nullable = true)
+    private final String mainOrganization;
 }