contributing.rdoc: double quotes [ci skip]

* doc/contributing.rdoc: double quotes to expand an environment
  variable.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51483 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

doc/contributing.rdoc

@@ -312,7 +312,7 @@ Now let's build CRuby:
     autoconf
     mkdir build && cd build # its good practice to build outside of source dir
     mkdir ~/.rubies # we will install to .rubies/ruby-trunk in our home dir
-    ../configure --prefix='$HOME/.rubies/ruby-trunk'
+    ../configure --prefix="$HOME/.rubies/ruby-trunk"
     make && make install
 
 After adding Ruby to your PATH, you should be ready to run the test suite: