Fix `make benchmark` example

`make benchmark ARGS=../benchmark/erb_render.yml` does not work.

```
% make benchmark ARGS=../benchmark/erb_render.yml
/Users/kamipo/.rbenv/shims/ruby --disable=gems -rrubygems -I./benchmark/lib ./benchmark/benchmark-driver/exe/benchmark-driver \
	            --executables="compare-ruby::/Users/kamipo/.rbenv/shims/ruby --disable=gems -I.ext/common --disable-gem" \
	            --executables="built-ruby::./miniruby -I./lib -I. -I.ext/common  ./tool/runruby.rb --extout=.ext  -- --disable-gems --disable-gem" \
	            ../benchmark/erb_render.yml 
Traceback (most recent call last):
	6: from ./benchmark/benchmark-driver/exe/benchmark-driver:112:in `<main>'
	5: from ./benchmark/benchmark-driver/exe/benchmark-driver:112:in `flat_map'
	4: from ./benchmark/benchmark-driver/exe/benchmark-driver:112:in `each'
	3: from ./benchmark/benchmark-driver/exe/benchmark-driver:122:in `block in <main>'
	2: from /Users/kamipo/.rbenv/versions/2.6.6/lib/ruby/2.6.0/psych.rb:577:in `load_file'
	1: from /Users/kamipo/.rbenv/versions/2.6.6/lib/ruby/2.6.0/psych.rb:577:in `open'
/Users/kamipo/.rbenv/versions/2.6.6/lib/ruby/2.6.0/psych.rb:577:in `initialize': No such file or directory @ rb_sysopen - ../benchmark/erb_render.yml (Errno::ENOENT)
make: *** [benchmark] Error 1

% make benchmark ARGS=benchmark/erb_render.yml
/Users/kamipo/.rbenv/shims/ruby --disable=gems -rrubygems -I./benchmark/lib ./benchmark/benchmark-driver/exe/benchmark-driver \
	            --executables="compare-ruby::/Users/kamipo/.rbenv/shims/ruby --disable=gems -I.ext/common --disable-gem" \
	            --executables="built-ruby::./miniruby -I./lib -I. -I.ext/common  ./tool/runruby.rb --extout=.ext  -- --disable-gems --disable-gem" \
	            benchmark/erb_render.yml 
Calculating -------------------------------------
                     compare-ruby  built-ruby 
          erb_render     825.454k    783.664k i/s -      1.500M times in 1.817181s 1.914086s

Comparison:
                       erb_render
        compare-ruby:    825454.4 i/s 
          built-ruby:    783663.8 i/s - 1.05x  slower

```

benchmark/README.md

@@ -61,7 +61,7 @@ make benchmark ITEM=vm OPTS=--filter=block
 
 # You can specify the benchmark by an exact filename instead of using the default argument:
 # ARGS = $$(find $(srcdir)/benchmark -maxdepth 1 -name '*$(ITEM)*.yml' -o -name '*$(ITEM)*.rb')
-make benchmark ARGS=../benchmark/erb_render.yml
+make benchmark ARGS=benchmark/erb_render.yml
 
 # You can specify any option via $OPTS
 make benchmark OPTS="--help"