lucet-runtime: fix doc test
//! let mut libc = PretendLibcRuntime { _unused: 0 };
//! let mut inst = region
//! .new_instance_builder(module)
-//! .with_embed_ctx(libc as *mut libc::c_void)
+//! .with_embed_ctx(libc)
//! .build()
//! .unwrap();
//!
