README: re-sync hello-world snippet after copyright add

Previous fix added a Copyright line to examples/hello_world.cpp to
satisfy cpplint, but scripts/check-readme.sh enforces that the first
```cpp fence in README.md matches the file byte-for-byte (TASK-041
A1 invariant). Add the same line to the snippet so check-readme
passes again. Verified locally with both check-readme.sh and
check-examples.sh (still 9 LOC, within the <=10 bound).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: etr

README.md

@@ -17,6 +17,7 @@ This README introduces the v2.0 API. It is not exhaustive: the headers under
 the authoritative reference.
 
 ```cpp
+// Copyright 2026 Sebastiano Merlino
 // libhttpserver hello-world example — the lambda form (PRD §3.4).
 // Compiles in ten lines including main(), with no http_resource subclass
 // and no raw-pointer ownership. Production code typically qualifies names