fix incorrect error message for deserialization in ValueSpecString (#1547)

backend/src/config/spec.rs

@@ -1213,7 +1213,7 @@ impl<'de> Deserialize<'de> for ValueSpecString {
                         masked = map.next_value()?;
                     } else if &key == "placeholder" {
                         if placeholder.is_some() {
-                            return Err(serde::de::Error::duplicate_field("pattern"));
+                            return Err(serde::de::Error::duplicate_field("placeholder"));
                         } else {
                             placeholder = Some(map.next_value()?);
                         }

backend/test/config-spec/lnd-correct.yaml

@@ -27,6 +27,7 @@ alias:
   name: Alias
   description: The public, human-readable name of your Lightning node
   nullable: true
+  placeholder: Enter a value
   pattern: ".{1,32}"
   pattern-description: Must be at least 1 character and no more than 32 characters
 color: