Update MMDB_read_node docs for >= bounds check

The implementation (maxminddb.c) uses >= to check node_number against
node_count, so passing a node_number equal to node_count is also an
error. Update the documentation to say "greater than or equal to"
instead of "greater than".

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: oschwald

doc/libmaxminddb.md

@@ -760,7 +760,7 @@ reference to an `MMDB_search_node_s` structure that will be populated by this
 function.
 
 The return value is a status code. If you pass a `node_number` that is greater
-than the number of nodes in the database, this function will return
+than or equal to the number of nodes in the database, this function will return
 `MMDB_INVALID_NODE_NUMBER_ERROR`, otherwise it will return `MMDB_SUCCESS`.
 
 The first node in the search tree is always node 0. If you wanted to iterate