doc: Correct typo in doc comments for volume/mounts/mounts/MountPoint

Signed-off-by: Jeongseok Kang <piono623@naver.com>

volume/mounts/mounts.go

@@ -61,7 +61,7 @@ type MountPoint struct {
 	// This should be set by calls to `Mount` and unset by calls to `Unmount`
 	ID string `json:",omitempty"`
 
-	// Sepc is a copy of the API request that created this mount.
+	// Spec is a copy of the API request that created this mount.
 	Spec mounttypes.Mount
 
 	// Some bind mounts should not be automatically created.