[doc] update MySQL version: 5.1.51 -> 5.5.13.
[doc] update MySQL version: 5.1.51 -> 5.5.13.

yoku0825/mroonga,naoa/mroonga,mroonga/mroonga,yoku0825/mroonga,yoku0825/mroonga,mroonga/mroonga,naoa/mroonga,naoa/mroonga,mroonga/mroonga,mroonga/mroonga,yoku0825/mroonga,naoa/mroonga,yoku0825/mroonga,naoa/mroonga,yoku0825/mroonga,naoa/mroonga