Change pyldap->python-ldap in the README

Follow-up to 5455333 and
bd69525.

The pyldap link pointed to an archived repository for a fork that has
been merged into mainline python-ldap package as of 3.0. This package
specifies python-ldap>=3.0.0 as dependency, so there's no need to
reference the out-of-date fork.

Author: Nickolay Ponomarev

README.md

@@ -13,7 +13,7 @@ First, install Flask-SimpleLDAP:
     $ pip install flask-simpleldap
 
 Flask-SimpleLDAP depends, and will install for you, recent versions of Flask
-(0.12.4 or later) and [pyldap](https://github.com/pyldap/pyldap). Flask-SimpleLDAP is compatible
+(0.12.4 or later) and [python-ldap](https://python-ldap.org/). Flask-SimpleLDAP is compatible
 with and tested on Python 3.5, 3.6 and 3.7.
 
 Next, add a ``LDAP`` instance to your code and at least the three