Fix replacement bug in gen-api-docs.sh

I introduced a bug in googleforgames#1621 that means that the publish/expiry
replacement would never happen.

This fixes that issue, while also allowing us to maintain the `set -e`
within the bash script.

site/gen-api-docs.sh

@@ -62,8 +62,7 @@ function sedeasy {
 }
 
 # do we have changes in generated API docs compared to previous version
-diff $RESULT $OLD || true
-if [ $? -gt 0 ]
+if ! diff $RESULT $OLD;
 then 
   echo "Output to a file $FILE"