docs(list): fix grammar in doc comment and README (#627)

Author: oahshtsua

list/README.md

@@ -17,7 +17,7 @@ type Item interface {
 ```
 
 ```go
-// DefaultItem describes an items designed to work with DefaultDelegate.
+// DefaultItem describes an item designed to work with DefaultDelegate.
 type DefaultItem interface {
 	Item
 	Title() string

list/defaultitem.go

@@ -61,7 +61,7 @@ func NewDefaultItemStyles() (s DefaultItemStyles) {
 	return s
 }
 
-// DefaultItem describes an items designed to work with DefaultDelegate.
+// DefaultItem describes an item designed to work with DefaultDelegate.
 type DefaultItem interface {
 	Item
 	Title() string