README.md: simplify sharded test command line

use xargs to keep the command in the foreground allowing it to be
stopped easily.

Change-Id: I6b4963b14fb6bff37542a3c5f7a23acc85f6a24d

README.md

diff --git a/README.md b/README.md
index 9de9c88..533a1da 100644
--- a/README.md
+++ b/README.md
@@ -249,11 +249,8 @@
    # Set the environment variable GTEST_TOTAL_SHARDS to 9 to run 10 test shards
    # (GTEST shard indexing is 0 based).
    $ export GTEST_TOTAL_SHARDS=9
-   $ for shard in $(seq 0 ${GTEST_TOTAL_SHARDS}); do \
-       [ ${shard} -lt ${GTEST_TOTAL_SHARDS} ] \
-         && GTEST_SHARD_INDEX=${shard} ./test_libaom & \
-     done
-
+   $ seq 0 $(( $GTEST_TOTAL_SHARDS - 1 )) \
+       | xargs -n 1 -P 0 -I{} env GTEST_SHARD_INDEX={} ./test_libaom
 ~~~
 
 To create a test shard for each CPU core available on the current system set