[doc] groonga -> mroonga.
[doc] groonga -> mroonga.

Reported by Tomoatsu Shimada. Thanks!!!

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