fix typo

Author: Abhi Keshav

README.rst

@@ -40,7 +40,7 @@ It is required to install Xcode command line tools, `homebrew <http://brew.sh>`_
    $ xcode-select --install
    $ /usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
    $ brew install python pkg-config libssh xml2 curl pcre cmake
-   $ curl -O https://devhub.cisco.com/artifactory/osx-ydk/0.6.1/libydk-0.6.1-Darwin.tar.gz
+   $ curl -O https://devhub.cisco.com/artifactory/osx-ydk/0.6.1/libydk-0.6.1-Darwin.pkg
    $ sudo installer -pkg libydk-0.6.1-Darwin.pkg -target /
 
 Windows