Update index.ngdoc

nodejs-legacy extra package is needed in Debian, for compatibility.

In http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=614907#108 you will find this comments:

1. The nodejs package shall be changed to provide /usr/bin/nodejs, not /usr/bin/node. The package should declare a Breaks: relationship with any packages in Debian that reference /usr/bin/node.

2. The nodejs source package shall also provide a nodejs-legacy binary package at Priority: extra that contains /usr/bin/node as a symlink to /usr/bin/nodejs. No package in the archive may depend on or recommend the nodejs-legacy package, which is provided solely for upstream
compatibility. This package declares shall also declare a Conflicts: relationship with the node package.