Docs: Stop calling LICENSE required in the user guide overview

The overview example still labelled the LICENSE declaration as required,
which contradicts the GPL default documented in decorators.md and the
quickstart.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: r41k0u

docs/user-guide/index.md

@@ -69,7 +69,7 @@ def my_function(ctx: c_void_p) -> c_int64:
     # BPF logic here
     return 0
 
-# License (required)
+# License (optional: without one, PythonBPF uses "GPL" and logs a warning)
 @bpf
 @bpfglobal
 def LICENSE() -> str: