Fix grammar in obscure error message (PaperMC#880)

The title really says it all. Change ``The forwarding-secret-file does not exists.`` to ``The forwarding-secret-file does not exist.``

proxy/src/main/java/com/velocitypowered/proxy/config/VelocityConfiguration.java

@@ -496,7 +496,7 @@ public static VelocityConfiguration read(Path path) throws IOException {
             throw new RuntimeException("The file " + forwardSecretFile + " is not a valid file or it is a directory.");
           }
         } else {
-          throw new RuntimeException("The forwarding-secret-file does not exists.");
+          throw new RuntimeException("The forwarding-secret-file does not exist.");
         }
       }
     }