README.md: Fix example by using an existing object file

The README.md would use a placeholder name for the object file in the
example, which is fine by itself but prevents us from validating the
example with "cargo test". Let's use an existing object file, so that
the program compiles and runs correctly.

Signed-off-by: Quentin Monnet <quentin@isovalent.com>

README.md

@@ -387,7 +387,7 @@ use rbpf::helpers;
 fn main() {
     // Load a program from an ELF file, e.g. compiled from C to eBPF with
     // clang/LLVM. Some minor modification to the bytecode may be required.
-    let filename = "my_ebpf_object_file.o";
+    let filename = "examples/load_elf__block_a_port.o";
 
     let path = PathBuf::from(filename);
     let file = match elf::File::open_path(&path) {