update readme file

Author: sandywalker

README.md

@@ -108,7 +108,7 @@ $('a').webuiPopover({
 	height:'auto',//can be set with  number
 	trigger:'click',//values:click,hover
 	style:'',//values:'',inverse
-	constrains:null, // constrains when placement is auto,  values: horizontal,vertical
+	constrains:null, // constrains the direction when placement is  auto,  values: horizontal,vertical
 	delay: {//show and hide delay time of the popover, works only when trigger is 'hover',the value can be number or object
 	    show: null,
 	    hide: 300