rpc/benchmark: rename main to simple

  Rename main package to simple since 'main' is too general
  as pointed at #390.
  And it doesn't seems to be good to integrate this with the existing
  benchmark client. So I renamed it for now until we find
  a better naming or structure.

Change-Id: Ie2559a5f2752b6578c8d2108cf7503b63847bf5c
diff --git a/runtime/internal/rpc/benchmark/README.md b/runtime/internal/rpc/benchmark/README.md
index b44ce9f..dd398f5 100644
--- a/runtime/internal/rpc/benchmark/README.md
+++ b/runtime/internal/rpc/benchmark/README.md
@@ -49,13 +49,13 @@
 
 `RESULTS.txt` has the full benchmark results.
 
-## `main/main.go`
+## `simple/main.go`
 
-main/main.go is a simple command-line tool to run the main benchmarks to measure
+`simple/main.go` is a simple command-line tool to run the main benchmarks to measure
 RPC setup time, latency, and throughput.
 
 ```
-$ v23 go run main/main.go
+$ v23 go run simple/main.go
 RPC Connection  33.48 ms/rpc
 RPC (echo 1000B)  1.31 ms/rpc (763.05 qps)
 RPC Streaming (echo 1000B)  0.11 ms/rpc
@@ -65,7 +65,7 @@
 # Client/Server
 ## `{benchmark,benchmarkd}/main.go`
 
-benchmarkd/main.go and benchmark/main.go are simple command-line tools to run the
+`benchmarkd/main.go` and `benchmark/main.go` are simple command-line tools to run the
 benchmark server and client as separate processes. Unlike the benchmarks above,
 this test includes the startup cost of name resolution, creating the VC, etc. in
 the first RPC.
diff --git a/runtime/internal/rpc/benchmark/main/main.go b/runtime/internal/rpc/benchmark/simple/main.go
similarity index 100%
rename from runtime/internal/rpc/benchmark/main/main.go
rename to runtime/internal/rpc/benchmark/simple/main.go