Fix the link to ProcMacro trait

Signed-off-by: Yuki Okushi <[email protected]>

Author: JohnTitor

src/macro-expansion.md

@@ -427,12 +427,12 @@ Some important data structures/interfaces here:
 - [`SyntaxExtensionKind`] - expander functions may have several different
   signatures (take one token stream, or two, or a piece of AST, etc). This is
   an enum that lists them.
-- [`ProcMacro`]/[`TTMacroExpander`]/[`AttrProcMacro`]/[`MultiItemModifier`] -
+- [`BangProcMacro`]/[`TTMacroExpander`]/[`AttrProcMacro`]/[`MultiItemModifier`] -
   traits representing the expander function signatures.
 
 [`SyntaxExtension`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_expand/base/struct.SyntaxExtension.html
 [`SyntaxExtensionKind`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_expand/base/enum.SyntaxExtensionKind.html
-[`ProcMacro`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_expand/base/trait.ProcMacro.html
+[`BangProcMacro`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_expand/base/trait.BangProcMacro.html
 [`TTMacroExpander`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_expand/base/trait.TTMacroExpander.html
 [`AttrProcMacro`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_expand/base/trait.AttrProcMacro.html
 [`MultiItemModifier`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_expand/base/trait.MultiItemModifier.html