doc: Fix broken markdown links

Detected by downstream ci job
https://cirrus-ci.com/task/6241220336287744?logs=lint#L459 in
https://corecheck.dev/bitcoin/bitcoin/pulls/31741

Author: ryanofsky

doc/usage.md

@@ -14,9 +14,9 @@ The `ProxyServer` objects help translate IPC requests from a socket to method ca
 
 ## Example
 
-A simple interface description can be found at [test/mp/test/foo.capnp](test/mp/test/foo.capnp), implementation in [test/mp/test/foo.h](test/mp/test/foo.h), and usage in [test/mp/test/test.cpp](test/mp/test/test.cpp).
+A simple interface description can be found at [test/mp/test/foo.capnp](../test/mp/test/foo.capnp), implementation in [test/mp/test/foo.h](../test/mp/test/foo.h), and usage in [test/mp/test/test.cpp](../test/mp/test/test.cpp).
 
-A more complete example can be found in [example](example/) and run with:
+A more complete example can be found in [example](../example/) and run with:
 
 ```sh
 make -C build example