Reflect new comment format in stringer.go

As of November 8th 2015, (4b07c5ce8a), the word "Code" is prepended to
the comments in Go source files generated by the stringer utility.

command/counthookaction_string.go

@@ -1,4 +1,4 @@
-// generated by stringer -type=countHookAction hook_count_action.go; DO NOT EDIT
+// Code generated by "stringer -type=countHookAction hook_count_action.go"; DO NOT EDIT
 
 package command
 

config/lang/ast/type_string.go

@@ -1,4 +1,4 @@
-// generated by stringer -type=Type; DO NOT EDIT
+// Code generated by "stringer -type=Type"; DO NOT EDIT
 
 package ast
 

helper/schema/getsource_string.go

@@ -1,4 +1,4 @@
-// generated by stringer -type=getSource resource_data_get_source.go; DO NOT EDIT
+// Code generated by "stringer -type=getSource resource_data_get_source.go"; DO NOT EDIT
 
 package schema
 

helper/schema/valuetype_string.go

@@ -1,4 +1,4 @@
-// generated by stringer -type=ValueType valuetype.go; DO NOT EDIT
+// Code generated by "stringer -type=ValueType valuetype.go"; DO NOT EDIT
 
 package schema
 

terraform/graphnodeconfigtype_string.go

@@ -1,4 +1,4 @@
-// generated by stringer -type=GraphNodeConfigType graph_config_node_type.go; DO NOT EDIT
+// Code generated by "stringer -type=GraphNodeConfigType graph_config_node_type.go"; DO NOT EDIT
 
 package terraform
 

terraform/instancetype_string.go

@@ -1,4 +1,4 @@
-// generated by stringer -type=InstanceType instancetype.go; DO NOT EDIT
+// Code generated by "stringer -type=InstanceType instancetype.go"; DO NOT EDIT
 
 package terraform
 

terraform/walkoperation_string.go

@@ -1,4 +1,4 @@
-// generated by stringer -type=walkOperation graph_walk_operation.go; DO NOT EDIT
+// Code generated by "stringer -type=walkOperation graph_walk_operation.go"; DO NOT EDIT
 
 package terraform