Update supported compiler in description of semver exempt polyfill

Author: dtolnay

README.md

@@ -62,7 +62,7 @@ proc-macro2 by default.
 
 To opt into the additional APIs available in the most recent nightly compiler,
 the `procmacro2_semver_exempt` config flag must be passed to rustc. We will
-polyfill those nightly-only APIs back to Rust 1.31.0. As these are unstable APIs
+polyfill those nightly-only APIs back to Rust 1.56.0. As these are unstable APIs
 that track the nightly compiler, minor versions of proc-macro2 may make breaking
 changes to them at any time.
 

src/lib.rs

@@ -65,7 +65,7 @@
 //!
 //! To opt into the additional APIs available in the most recent nightly
 //! compiler, the `procmacro2_semver_exempt` config flag must be passed to
-//! rustc. We will polyfill those nightly-only APIs back to Rust 1.31.0. As
+//! rustc. We will polyfill those nightly-only APIs back to Rust 1.56.0. As
 //! these are unstable APIs that track the nightly compiler, minor versions of
 //! proc-macro2 may make breaking changes to them at any time.
 //!