Typo fixed and simple code.

Signed-off-by: Ri Xu <xuri@360.net>

rewritten from github.com/moby/moby 87e8a936e82ecfbef59b829d8bbfca3fb4aa3163

frontend/dockerfile/instructions/commands.go

@@ -9,7 +9,7 @@ import (
 	"github.com/docker/docker/api/types/strslice"
 )
 
-// KeyValuePair represent an arbitrary named value (usefull in slice insted of map[string] string to preserve ordering)
+// KeyValuePair represent an arbitrary named value (useful in slice insted of map[string] string to preserve ordering)
 type KeyValuePair struct {
 	Key   string
 	Value string

frontend/dockerfile/instructions/parse.go

@@ -105,7 +105,7 @@ func ParseCommand(node *parser.Node) (Command, error) {
 	return nil, errors.Errorf("%T is not a command type", s)
 }
 
-// UnknownInstruction represents an error occuring when a command is unresolvable
+// UnknownInstruction represents an error occurring when a command is unresolvable
 type UnknownInstruction struct {
 	Line        int
 	Instruction string