Make process corpus field read only in Django admin

arkindex/process/admin.py

@@ -43,7 +43,7 @@ class ProcessAdmin(admin.ModelAdmin):
             "fields": ("element", "element_type", "folder_type", "name_contains")
         }),
     )
-    readonly_fields = ("id", "name", "state", "activity_state", "template")
+    readonly_fields = ("id", "name", "corpus", "state", "activity_state", "template")
     raw_id_fields = ("element", )
     inlines = [DataFileInline, WorkerRunInline, ElementInline]