README: remove outdated typo

Author: tylerwhall

README.rst

@@ -52,8 +52,7 @@ Rust toolchain
 
 A nightly compiler is required to use unstable features that are unavoidable
 when implementing libstd. The nightly date is arbitrary, but needs to be locked
-so long as we are using rust-src from rustup. The interface between std and
-core is not stable so we need to look the version for build reproducibility.
+so long as we are using rust-src from rustup.
 
 .. code-block:: console