usage.md: specify bash for code block semantics

tests/usage.md

@@ -8,19 +8,19 @@
 * Wrk
 
 ## Latency test
-```
+```bash
 ./stress-latency-60s
 ```
 This will run an latency benchmark with Vegeta for 60s by default and output plot.html.
 
 ## Throughput test
-```
+```bash
 ./stress-throughput-60s
 ```
 This script run an throughtput benchmark using Wrk for 60 by default and outputs the results in your terminal.
 
 ## Endurance test
-```
+```bash
 PROFILING=true ./stress -add -n 1000000
 ```
 It will bring up the server and execute and insert 1M events with default concurrency of 10 connections. Also will start a go profiling tool to collect all internal data of QED server an save it in 'results' directory.
@@ -30,6 +30,6 @@ The graph is saved in 'results/graph-Add-$number_events.png (results/graph-Add-1
 This script use 'riot.go'. It's a tool we've develop to stress QED. It's also capable of generating charts, start with specific offset or even run Membership|Incremental Proofs for the events into QED.
 
 If you install Dialog and Gnuplot you can generate usage graphs for each function collected by Golang pprof tool. ei:
-```
+```bash
 ./make_graph results
 ```