docs: fix raft_node_get_match_idx's comment

[debug-zhang]

In include/raft.h

The function raft_node_get_match_idx's comment is wrong:

/**
 * @return this node's user data */
raft_index_t raft_node_get_match_idx(raft_node_t* me);

It should be:

/**
 * @return this node's match index */
raft_index_t raft_node_get_match_idx(raft_node_t* me);