Increase heading size

Currently the main heading of the readme uses a level three markdown
heading, this is quite small. Use level two instead.

Author: tcharding

README.md

@@ -2,7 +2,7 @@
 
 [Full documentation](https://docs.rs/secp256k1/)
 
-### rust-secp256k1
+## rust-secp256k1
 
 `rust-secp256k1` is a wrapper around [libsecp256k1](https://github.com/bitcoin-core/secp256k1),
 a C library by Pieter Wuille for producing ECDSA signatures using the SECG curve