Remove spaces in --only-group flag documentation (#795)

optparse documentation says that Array type flags are "Strings separated
by ',' (e.g. 1,2,3)". And indeed `--only-group 1, 2, 3` only tests group
1.

https://ruby-doc.org/stdlib-2.6.6/libdoc/optparse/rdoc/OptionParser.html

Readme.md

@@ -210,7 +210,7 @@ Options are:
     -i, --isolate                    Do not run any other tests in the group used by --single(-s).
                                      Automatically turned on if --isolate-n is set above 0.
         --isolate-n                  Number of processes for isolated groups. Default to 1 when --isolate is on.
-        --only-group INT[, INT]
+        --only-group INT[,INT]
     -e, --exec [COMMAND]             execute this code parallel and with ENV['TEST_ENV_NUMBER']
     -o, --test-options '[OPTIONS]'   execute test commands with those options
     -t, --type [TYPE]                test(default) / rspec / cucumber / spinach

lib/parallel_tests/cli.rb

@@ -208,7 +208,7 @@ def parse_options!(argv)
           options[:isolate_count] = n
         end
 
-        opts.on("--only-group INT[, INT]", Array) { |groups| options[:only_group] = groups.map(&:to_i) }
+        opts.on("--only-group INT[,INT]", Array) { |groups| options[:only_group] = groups.map(&:to_i) }
 
         opts.on("-e", "--exec [COMMAND]", "execute this code parallel and with ENV['TEST_ENV_NUMBER']") { |path| options[:execute] = path }
         opts.on("-o", "--test-options '[OPTIONS]'", "execute test commands with those options") { |arg| options[:test_options] = arg.lstrip }