Fix relative links in the benchmark app README.

R=qsr@google.com, qsr@chromium.org

Review URL: https://codereview.chromium.org/1309323003 .
diff --git a/apps/benchmark/README.md b/apps/benchmark/README.md
index 60d76b7..ca78054 100644
--- a/apps/benchmark/README.md
+++ b/apps/benchmark/README.md
@@ -4,7 +4,7 @@
 indicated period of time and computes a number of results based on the collected
 traces. It can be used to measure performance of a mojo app, provided that the
 app being benchmarked participates in the [tracing
-ecosystem](mojo/services/tracing/public/interfaces/tracing.mojom).
+ecosystem](../../mojo/services/tracing/public/interfaces/tracing.mojom).
 
 ## Arguments
 
@@ -26,5 +26,5 @@
 
 ## Runner script
 
-Devtools offers [a helper script](mojo/devtools/common/mojo_benchmark) allowing
-to run a list of benchmarks, including on an Android device.
+Devtools offers [a helper script](../../mojo/devtools/common/mojo_benchmark)
+allowing to run a list of benchmarks, including on an Android device.