API-Rust.md: Fix section numbering

Must of got out of whack when editing it from the C version.

Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

Author: ac000

API-Rust.md

@@ -40,7 +40,7 @@ and there isn't a real need to create wrappers specifically for them.
   * [Version](#version)
   * [String Conversion](#string-conversion)
   * [uwr_write_str!](#uwr_write_str)
-6. [Function Handlers](#function-handlers)
+3. [Function Handlers](#function-handlers)
   * [Optional](#optional)
     - [luw_module_init_handler](#luw_module_init_handler)
     - [luw_module_end_handler](#luw_module_end_handler)
@@ -51,7 +51,7 @@ and there isn't a real need to create wrappers specifically for them.
     - [luw_request_handler](#luw_request_handler)
     - [luw_free_handler](#luw_free_handler)
     - [luw_malloc_handler](#luw_malloc_handler)
-7. [Functions](#functions)
+4. [Functions](#functions)
   * [UWR_CTX_INITIALIZER](#uwr_ctx_initializer)
   * [uwr_init_ctx](#uwr_init_ctx)
   * [uwr_set_req_buf](#uwr_set_req_buf)
@@ -84,7 +84,7 @@ and there isn't a real need to create wrappers specifically for them.
   * [uwr_http_send_headers](#uwr_http_send_headers)
   * [uwr_http_response_end](#uwr_http_response_end)
   * [uwr_mem_get_init_size](#uwr_mem_get_init_size)
-8. [Misc. Functions](#misc-functions)
+5. [Misc. Functions](#misc-functions)
   * [uwr_malloc](#uwr_malloc)
   * [uwr_free](#uwr_free)