runtime_config: Replace 'MountPoints in Spec' with 'Spec.Mounts'

I think Go's attribute syntax reads more clearly here, especially
since there is no 'Spec.MountPoints' after c18c283 (Change layout of
mountpoints and mounts, 2015-09-02, opencontainers#136).

Signed-off-by: W. Trevor King <wking@tremily.us>

runtime_config.go

@@ -2,8 +2,7 @@ package specs
 
 type RuntimeSpec struct {
 	// Mounts is a mapping of names to mount configurations.
-	// Which mounts will be mounted and where should be chosen with MountPoints
-	// in Spec.
+	// Which mounts will be mounted and where should be chosen with Spec.Mounts.
 	Mounts map[string]Mount `json:"mounts"`
 	// Hooks are the commands run at various lifecycle events of the container.
 	Hooks Hooks `json:"hooks"`