[IO-776] Fix parameters to requireNonNull call

Currently, the call checks that the literal string `"prefix"` is
non-null, which of course it is :)

Author: cpovirk

src/main/java/org/apache/commons/io/output/DeferredFileOutputStream.java

@@ -141,7 +141,7 @@ public DeferredFileOutputStream(final int threshold, final int initialBufferSize
     public DeferredFileOutputStream(final int threshold, final int initialBufferSize, final String prefix,
         final String suffix, final File directory) {
         this(threshold, null, prefix, suffix, directory, initialBufferSize);
-        Objects.requireNonNull("prefix", prefix);
+        Objects.requireNonNull(prefix, "prefix");
         if (initialBufferSize < 0) {
             throw new IllegalArgumentException("Initial buffer size must be at least 0.");
         }