README: Use absolute link for image

crates.io doesn't handle relative links well:
rust-lang/crates.io#9939 (comment)

It is a bit sad to do this, because now you need internet to render the
README locally;

OTOH if we ever decide to change the logo, the new one will be
automatically visible in any place that uses absolute link.

README.md

@@ -1,4 +1,4 @@
-<img src="assets/monster+rust.png" height="150" align="right">
+<img src="https://github.com/scylladb/scylla-rust-driver/raw/main/assets/monster+rust.png" height="150" align="right">
 
 # ScyllaDB Rust Driver