sync run nightly install directions with Book

Signed-off-by: Dmitry Savintsev <dsavints@gmail.com>

README.md

@@ -3,7 +3,7 @@
 ## Prerequisites
 
 1. Install a rust stable toolchain: `rustup install stable`
-1. Install a rust nightly toolchain: `rustup install nightly`
+1. Install a rust nightly toolchain with the rust-src component: `rustup toolchain install nightly --component rust-src`
 1. Install bpf-linker: `cargo install bpf-linker`
 
 ## Build eBPF