[FAB-4439] Fix typo in README.md for configtxlator

Configtxlator doesn't start by following README.md
(example/configtxupdate/README.md).
The instruction is missing an arg to start configtxlator.

This patch fixes the document.

Change-Id: If9669398216dd51f8a0a360035dec4000023922a
Signed-off-by: Yuki Kondo <yuki.kondo@hal.hitachi.com>

Author: yuki-kon

examples/configtxupdate/README.md

@@ -68,7 +68,7 @@ CGO_CFLAGS=" " GOBIN=/home/yellickj/go/src/github.com/hyperledger/fabric/build/b
 Binary available as build/bin/configtxlator
 ```
 ```
-$ configtxlator
+$ configtxlator start
 2017-05-31 12:57:22.499 EDT [configtxlator] main -> INFO 001 Serving HTTP requests on port: 7059
 ```
 
@@ -124,7 +124,7 @@ CGO_CFLAGS=" " GOBIN=/home/yellickj/go/src/github.com/hyperledger/fabric/build/b
 Binary available as build/bin/configtxlator
 ```
 ```
-$ configtxlator
+$ configtxlator start
 2017-05-31 12:57:22.499 EDT [configtxlator] main -> INFO 001 Serving HTTP requests on port: 7059
 ```