Fix README.md to include "tang" pin (latchset#424)

All references to "http" pin make no sense in README.md
or other documentation, as "http" pin exists no more.
They must be changed to "tang" pin. This commit changes
references to "http" pin and substitute them to "tang"

Resolves: latchset#423

Signed-off-by: Sergio Arroutbi <[email protected]>

README.md

@@ -102,16 +102,16 @@ In the above example, we define two child pins and have a threshold of 2.
 This means that during decryption **both** child pins must succeed in order for
 SSS itself to succeed.
 
-Here is another example where we use just the HTTP pin:
+Here is another example where we use just the Tang pin:
 
 ```bash
 $ echo hi | clevis encrypt sss \
-'{"t": 1, "pins": {"http": [{"url": "http://server1.local/key"}, {"url": "http://server1.local/key"}]}}' \
+'{"t": 1, "pins": {"tang": [{"url": "http://server1.local/key"}, {"url": "http://server2.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
+In this example, we define two child instances of the Tang pin - each with its
+own configuration. Since we have a threshold of 1, if **either** of the Tang
 pin instances succeed during decryption, SSS will succeed.
 
 ### Binding LUKS Volumes