Remove unencrypted git protocol use (#1100)

Because Github removes support for unencrypted git protocol in March 2022, URLs in scripts are changed to use https.

See https://github.blog/2021-09-01-improving-git-protocol-security-github/ for details

INSTALL

@@ -51,7 +51,7 @@ like to contribute an installation script, we would welcome it!)
    If you're reading this, you've probably already done so, but the
    command to download the Mininet source code is:
 
-        git clone git://github.com/mininet/mininet.git
+        git clone https://github.com/mininet/mininet.git
 
    Note that the above git command will check out the latest and greatest
    Mininet (which we recommend!) If you want to run the last
@@ -147,7 +147,7 @@ like to contribute an installation script, we would welcome it!)
 
     * clone the Mininet repository
 
-        git clone git://github.com/mininet/mininet.git
+        git clone https://github.com/mininet/mininet.git
 
     * install Mininet, the OpenFlow reference implementation, and
       Open vSwitch