doc: Update NetBSD Build Guide

The `boost-headers` package does not provide CMake configuration files
required since bitcoin#32667. Install
the `boost` package instead.

Author: hebasto

doc/build-netbsd.md

@@ -12,7 +12,7 @@ Install the required dependencies the usual way you [install software on NetBSD]
 The example commands below use `pkgin`.
 
 ```bash
-pkgin install git cmake pkg-config boost-headers libevent
+pkgin install git cmake pkg-config boost libevent
 ```
 
 NetBSD currently ships with an older version of `gcc` than is needed to build. You should upgrade your `gcc` and then pass this new version to the configure script.