doc(core): remove a broken intra-doc link to crate#macros

Author: yvt

src/r3_core/src/lib.md

@@ -27,7 +27,6 @@ The core API represents the low-level interface between kernel and application c
 - [Stability](#stability)
 - [Cargo Features](#cargo-features)
 - [Modules](#modules)  <!-- this section is generated by rustdoc -->
-- [Macros](#macros)  <!-- this section is generated by rustdoc -->
 
 # Note to Application Developers