Fix some missing emdashes

These list entries were missing an emdash like the rest of the entries.

Author: ehuss

src/attributes.md

@@ -293,8 +293,8 @@ The following is an index of all built-in attributes.
   - [`cold`] --- Hint that a function is unlikely to be called.
   - [`no_builtins`] --- Disables use of certain built-in functions.
   - [`target_feature`] --- Configure platform-specific code generation.
-  - [`track_caller`] - Pass the parent call location to `std::panic::Location::caller()`.
-  - [`instruction_set`] - Specify the instruction set used to generate a functions code
+  - [`track_caller`] --- Pass the parent call location to `std::panic::Location::caller()`.
+  - [`instruction_set`] --- Specify the instruction set used to generate a functions code
 
 - Documentation
   - `doc` --- Specifies documentation. See [The Rustdoc Book] for more