Fix arg types of et#.

Doc/c-api/arg.rst

@@ -136,7 +136,7 @@ of the C variable(s) whose address should be passed.
    In both cases, *\*buffer_length* is set to the length of the encoded data
    without the trailing NUL byte.
 
-``et#`` (string, Unicode object or character buffer compatible object) [const char \*encoding, char \*\*buffer]
+``et#`` (string, Unicode object or character buffer compatible object) [const char \*encoding, char \*\*buffer, int \*buffer_length]
    Same as ``es#`` except that string objects are passed through without
    recoding them. Instead, the implementation assumes that the string object
    uses the encoding passed in as parameter.