Allow copy / paste from the readme

Remove the $ in the shell commands and wrapping them in code blocks
allows for copy/paste directly into a terminal from GitHub.

Signed-off-by: Tim Smith <tsmith84@gmail.com>

README.md

@@ -14,15 +14,18 @@ gem 'resolv'
 
 And then execute:
 
-    $ bundle install
+```bash
+bundle install
+```
 
 Or install it yourself as:
 
-    $ gem install resolv
+```bash
+gem install resolv
+```
 
 ## Usage
 
-
 ```ruby
 p Resolv.getaddress "www.ruby-lang.org"
 p Resolv.getname "210.251.121.214"
@@ -44,4 +47,3 @@ To install this gem onto your local machine, run `bundle exec rake install`. To
 ## Contributing
 
 Bug reports and pull requests are welcome on GitHub at https://github.com/ruby/resolv.
-