Correct README.md to not include "http" pin, as it exists no more

[sarroutbi]

README.md documentation states:

Here is another example where we use just the HTTP pin:

$ echo hi | clevis encrypt sss \
'{"t": 1, "pins": {"http": [{"url": "http://server1.local/key"}, {"url": "http://server1.local/key"}]}}' \
> hi.jwe
In this example, we define two child instances of the HTTP pin - each with its own configuration. Since we have a threshold of 1, if either of the HTTP pin instances succeed during decryption, SSS will succeed.

This should be rewritten to use "tang" instead of "http" pin, as this pin doesn't exist any more