fix exception message

Signed-off-by: Lukas Jungmann <lukas.jungmann@oracle.com>

api/src/main/java/jakarta/json/spi/JsonProvider.java

diff --git a/api/src/main/java/jakarta/json/spi/JsonProvider.java b/api/src/main/java/jakarta/json/spi/JsonProvider.java
index 02ca904..4587f77 100644
--- a/api/src/main/java/jakarta/json/spi/JsonProvider.java
+++ b/api/src/main/java/jakarta/json/spi/JsonProvider.java
@@ -158,10 +158,10 @@
             return clazz.getConstructor().newInstance();
         } catch (ClassNotFoundException x) {
             throw new JsonException(
-                    "Provider " + DEFAULT_PROVIDER + " not found", x);
+                    "Provider " + className + " not found", x);
         } catch (Exception x) {
             throw new JsonException(
-                    "Provider " + DEFAULT_PROVIDER + " could not be instantiated: " + x,
+                    "Provider " + className + " could not be instantiated: " + x,
                     x);
         }
     }