swagger: fix "generated code" comment not in correct format

As described in https://golang.org/s/generatedcode, Go has
a formalized format that should be used to indicate that a
file is generated.

Matching that format helps linters to skip generated files;

From https://golang.org/s/generatedcode (https://github.com/golang/go/issues/13560#issuecomment-288457920);

> Generated files are marked by a line of text that matches the regular expression, in Go syntax:
>
>     ^// Code generated .* DO NOT EDIT\.$
>
> The `.*` means the tool can put whatever folderol it wants in there, but the comment
> must be a single line and must start with `Code generated` and end with `DO NOT EDIT.`,
> with a period.
>
> The text may appear anywhere in the file.

This patch updates the template used for our generated types
to match that format.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

api/templates/server/operation.gotmpl

@@ -1,8 +1,7 @@
 package {{ .Package }} // import "github.com/docker/docker/api/types/{{ .Package }}"
 
 // ----------------------------------------------------------------------------
-// DO NOT EDIT THIS FILE
-// This file was generated by `swagger generate operation`
+// Code generated by `swagger generate operation`. DO NOT EDIT.
 //
 // See hack/generate-swagger-api.sh
 // ----------------------------------------------------------------------------