remove an from log message

Co-authored-by: Andre_601 <11576465+Andre601@users.noreply.github.com>

src/main/java/me/clip/placeholderapi/expansion/manager/LocalExpansionManager.java

@@ -340,7 +340,7 @@ public final class LocalExpansionManager implements Listener {
 
         if (expansionClass == null) {
           plugin.getLogger().severe("Failed to load Expansion: " + file.getName() + ", as it does not have" +
-                  "an a class which extends PlaceholderExpansion.");
+                  " a class which extends PlaceholderExpansion.");
         }
 
         return expansionClass;