fix html characters in virtualbox builder doc

This commit removes <, > and instead uses <,> to describe the
enter command in the boot command code block in virtualbox
builder documentation. Noticed this after fixing the VMware doc.

website/source/docs/builders/virtualbox.html.markdown

@@ -191,7 +191,7 @@ an Ubuntu 12.04 installer:
   "fb=false debconf/frontend=noninteractive ",
   "keyboard-configuration/modelcode=SKIP keyboard-configuration/layout=USA ",
   "keyboard-configuration/variant=USA console-setup/ask_detect=false ",
-  "initrd=/install/initrd.gz -- <enter>"
+  "initrd=/install/initrd.gz -- &lt;enter&gt;"
 ]
 </pre>