Fix README to require correct module name

[rynop]

Description

Module name does not match what is documented:

cat node_modules/@node-saml/passport-saml/package.json
{
  "name": "@node-saml/passport-saml",

Checklist:

• Issue Addressed: [ ]
• Link to SAML spec: [ ]
• Tests included? [ ]
• Documentation updated? [x]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 64.42%. Comparing base (30b1478) to head (0f0ff67).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #884   +/-   ##
=======================================
  Coverage   64.42%   64.42%           
=======================================
  Files           4        4           
  Lines         149      149           
  Branches       37       37           
=======================================
  Hits           96       96           
  Misses         30       30           
  Partials       23       23           

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