fix: WASM32 target needed for stable Linux build

[Rqnsom]

After following the instructions, the build fails because:

user@host:~/substrate-node-template$ cargo build --release
   Compiling node-template-runtime v4.0.0-dev (/home/user/substrate-node-template/runtime)
error: failed to run custom build command for `node-template-runtime v4.0.0-dev (/home/user/substrate-node-template/runtime)`

Caused by:
  process didn't exit successfully: `/home/user/substrate-node-template/target/release/build/node-template-runtime-5c6b7b372ef575b9/build-script-build` (exit status: 1)
  --- stderr
  Rust WASM toolchain not installed, please install it!

Similarly to the recent commit on Mac, the same line needs to be added for the Linux build instructions.

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[Rqnsom]

@Rashmirreddy, I saw you approved a few recent PRs.
May I ask here how I can get this PR approved? Should I create an issue for it? I don't see any contribution instructions.