Update installation.md

[sgt101]

removed the date mention to reflect that it builds with the current nightly

[awdemos]

Thanks for the docs update — dropping the pinned nightly date makes the install instructions less fragile.

One small question: since Scallop still uses unstable nightly features, should we add a note that any recent nightly is required, or link to a rust-toolchain.toml if the repo has one? Otherwise this looks good to merge.