Update Gauge.java (#432)

Comment is Wrong

Signed-off-by: Jue Chen <3330584+jchprj@users.noreply.github.com>

simpleclient/src/main/java/io/prometheus/client/Gauge.java

@@ -254,7 +254,7 @@ public void inc(double amt) {
     noLabelsChild.inc(amt);
   }
   /**
-   * Increment the gauge with no labels by 1.
+   * Decrement the gauge with no labels by 1.
    */
   public void dec() {
     dec(1);