fix error message

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1893170 13f79535-47bb-0310-9956-ffa450edef68

poi-ooxml/src/main/java/org/apache/poi/xslf/util/PPTX2PNG.java

@@ -272,7 +272,7 @@ public final class PPTX2PNG {
         }
 
         if (!"long,short,width,height,scale".contains(fixSide)) {
-            usage("<fixside> must be one of long / short / width / height");
+            usage("<fixside> must be one of long / short / width / height / scale");
             return false;
         }