fix misspelling issue

context.go

@@ -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