drm/sti: Fix a typo in a comment line

Add a missing character in this description for a data structure.

Signed-off-by: Markus Elfring <[e-mail]>
Signed-off-by: Benjamin Gaignard <[e-mail]>
Link: http://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/sti/sti_cursor.c

···
 #define STI_CURS_MAX_SIZE   128
 
 /*
- * pixmap dma buffer stucture
+ * pixmap dma buffer structure
  *
  * @paddr:  physical address
  * @size:   buffer size