fix misspelling issue
@@ -73,7 +73,7 @@ func (c *Context) IsSet(name string) bool {
// change in version 2 to add `IsSet` to the Flag interface to push the
// responsibility closer to where the information required to determine
// whether a flag is set by non-standard means such as environment
- // variables is avaliable.
+ // variables is available.
//
// See https://github.com/urfave/cli/issues/294 for additional discussion
flags := c.Command.Flags
