[Rspamd] Decrease weight of missed charset

data/conf/rspamd/local.d/headers_group.conf

@@ -0,0 +1,7 @@
+symbols = {
+  "R_MIXED_CHARSET" {
+    weight = 1.0;
+    description = "Mixed characters in a message";
+    one_shot = true;
+  }
+}