[propose] fix some misspell errors

[hustjieke]

I found some misspell errors like, we can fix them:

xenon/src/model/mysql.go
Line 31: warning: "postion" is a misspelling of "position" (misspell)
xenon/src/raft/candidate.go
Line 64: warning: "boradcast" is a misspelling of "broadcast" (misspell)
Line 86: warning: "boradcast" is a misspelling of "broadcast" (misspell)
xenon/src/raft/mock.go
Line 135: warning: "boradcast" is a misspelling of "broadcast" (misspell)
Line 149: warning: "boradcast" is a misspelling of "broadcast" (misspell)
xenon/src/raft/raft_test.go
Line 254: warning: "boradcast" is a misspelling of "broadcast" (misspell)
...
...

[BohuTANG]

ACK