fix the last link

Author: mark-i-m

src/profiling/with_perf.md

@@ -14,7 +14,7 @@ This is a guide for how to profile rustc with [perf](https://perf.wiki.kernel.or
 - Make a rustup toolchain pointing to that result
   - see [the "build and run" section for instructions][b-a-r]
 
-[b-a-r]: ./how-to-build-and-run.html#toolchain
+[b-a-r]: ../how-to-build-and-run.html#toolchain
 
 ## Gathering a perf profile