Update comment of AlterTableCmd->name in parsenodes.h

Since b0483263dd, this field can be used to store an access method
name for ALTER TABLE, but access methods were not mentioned in the
field's description.

Issue noticed while working on the area.

Discussion: https://postgr.es/m/ZeWKgCtk6xiAsDsc@paquier.xyz

src/include/nodes/parsenodes.h

@@ -2277,7 +2277,7 @@ typedef struct AlterTableCmd	/* one subcommand of an ALTER TABLE */
 	NodeTag		type;
 	AlterTableType subtype;		/* Type of table alteration to apply */
 	char	   *name;			/* column, constraint, or trigger to act on,
-								 * or tablespace */
+								 * or tablespace, access method */
 	int16		num;			/* attribute number for columns referenced by
 								 * number */
 	RoleSpec   *newowner;