Correct the .encode_abi example web3.contract.rst

Previously, fn_name was a named argument, used like:

```
contract.encode_abi(fn_name="register", ...)
```

But now, fn_name is the unnamed first argument, used like:

```
contract.encode_abi("register", ...)
```

(I believe, at least - I note that line 326, which defines the class method, uses it this way:

```
.. py:classmethod:: Contract.encode_abi(fn_name, args=None, kwargs=None, data=None)
```

And in my local project `encode_abi("getNames", ...)` worked where `encode_abi(fn_name="getNames", ...)` did not.

Author: flanagansteve

docs/web3.contract.rst

@@ -331,7 +331,7 @@ Each Contract Factory exposes the following methods.
 
     .. code-block:: python
 
-      >>> contract.encode_abi(fn_name="register", args=["rainbows", 10])
+      >>> contract.encode_abi("register", args=["rainbows", 10])
       "0xea87152b0000000000000000000000000000000000000000000000000000000000000040000000000000000000000000000000000000000000000000000000000000000a00000000000000000000000000000000000000000000000000000000000000087261696e626f7773000000000000000000000000000000000000000000000000"