Tiny change to README to suggest using 'system' over 'exec'

Reason being, 'exec' will run the shell command and then exit the ruby
process. While 'system will run the shell command and then continue
execution.

I feel this is a better example as the first thing I did was write a
rake task that chains multiple tasks together and the way 'exec' was
ending the process cost me a bit of time trouble shooting why.

README.md

@@ -137,7 +137,7 @@ end
 namespace :deploy do
   def deploy(env)
     puts "Deploying to #{env}"
-    exec "TARGET=#{env} bundle exec middleman deploy"
+    system "TARGET=#{env} bundle exec middleman deploy"
   end
 
   task :staging do