benchmark/README.md: skip showing --timeout option [ci skip]

because it's only available for limited platforms for now.
I'll make it portable and show it later.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63954 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

benchmark/README.md

@@ -37,7 +37,6 @@ Usage: benchmark-driver [options] RUBY|YAML...
         --bundler                    Install and use gems specified in Gemfile
         --filter REGEXP              Filter out benchmarks with given regexp
         --run-duration SECONDS       Warmup estimates loop_count to run for this duration (default: 3)
-        --timeout SECONDS            Timeout ruby command execution with timeout(1)
     -v, --verbose                    Verbose mode. Multiple -v options increase visilibity (max: 2)
 ```