Corrected exception message in UPCEWriter (#1074)

- according to EAN13Writer, both possibilities should be reflected within the message

core/src/main/java/com/google/zxing/oned/UPCEWriter.java

@@ -73,7 +73,7 @@ public final class UPCEWriter extends UPCEANWriter {
         break;
       default:
         throw new IllegalArgumentException(
-            "Requested contents should be 8 digits long, but got " + length);
+            "Requested contents should be 7 or 8 digits long, but got " + length);
     }
 
     checkNumeric(contents);