minor readme update

Author: ultd

readme.md

@@ -123,7 +123,7 @@ Add `Sig` to your Zig project using `build.zig.zon` file (available for Zig >= 0
 
 ## Usage
 
-### `Pubkey` - API Reference
+### `core.Pubkey` - API Reference
 
 A struct which holds a Public Key of a Solana account (`[32]u8`).