Update widget page to describe correct getter function

Previously page described use of option method as:

```
var bar = $( "<div />")
    .appendTo( "body" )
    .progressbar()
    .data( "progressbar" );
 
bar.option( "value"); // Setter
```

This correction describes the usage of the option method as:
```
var bar = $( "<div />")
    .appendTo( "body" )
    .progressbar()
    .data( "progressbar" );
 
bar.option( "value"); // Getter
```