fix typo

Author: Abhi Keshav

README.rst

@@ -41,7 +41,7 @@ You can download the latest python package from `here <https://www.python.org/do
    $ xcode-select --install
    $ /usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
    $ brew install pkg-config libssh xml2 libxml2 curl pcre cmake
-   $ curl -O https://devhub.cisco.com/artifactory/osx-ydk/0.6.3/libydk-0.6.3-Darwin.tar.gz
+   $ curl -O https://devhub.cisco.com/artifactory/osx-ydk/0.6.3/libydk-0.6.3-Darwin.pkg
    $ sudo installer -pkg libydk-0.6.3-Darwin.pkg -target /
 
 Windows