Make samply installation command better

Author: Kobzol

src/bootstrap/src/core/build_steps/perf.rs

@@ -30,7 +30,7 @@ enum PerfCommand {
     Eprintln,
     /// Run `profile_local samply`
     /// This executes the compiler on the given benchmarks and profiles it with `samply`.
-    /// You need to install `samply`, e.g. using `cargo install samply`.
+    /// You need to install `samply`, e.g. using `cargo install --locked samply`.
     Samply,
     /// Run `profile_local cachegrind`.
     /// This executes the compiler on the given benchmarks under `Cachegrind`.