Correct a small spelling mistake.

session.go

@@ -1574,7 +1574,7 @@ var (
 	ErrUnavailable          = errors.New("unavailable")
 	ErrUnsupported          = errors.New("feature not supported")
 	ErrTooManyStmts         = errors.New("too many statements")
-	ErrUseStmt              = errors.New("use statements aren't supported. Please see https://github.com/gocql/gocql for explaination.")
+	ErrUseStmt              = errors.New("use statements aren't supported. Please see https://github.com/gocql/gocql for explanation.")
 	ErrSessionClosed        = errors.New("session has been closed")
 	ErrNoConnections        = errors.New("gocql: no hosts available in the pool")
 	ErrNoKeyspace           = errors.New("no keyspace provided")