README: refer to `libsecp256k1-dev` instead of `libsecp256k1-0`

We don't actually need the development headers, instead using this as
a hack to be agnostic to the version scheme and pull in the latest.

related:
https://github.com/spesmilo/electrum/pull/8185
https://github.com/spesmilo/electrum/pull/8320
https://github.com/spesmilo/electrum/issues/8328#issuecomment-1518061250

debian 11 (stable) only has libsecp256k1-0
debian 12 (testing) atm only has libsecp256k1-1
ubuntu 23.04 only has libsecp256k1-1
I expect libsecp256k1-2 might soon get packaged too, now that upstream secp released v0.3.0.
So what do we tell users to install? well, turns out most distros have libsecp256k1-dev, which
just pulls in the latest secp.
Caveat: if there is a new secp release that actually gets packaged on a distro before we can react,
then this new instruction will not work.

README.md

@@ -22,7 +22,7 @@ but not everything. The following sections describe how to run from source, but
 is a TL;DR:
 
 ```
-$ sudo apt-get install libsecp256k1-0
+$ sudo apt-get install libsecp256k1-dev
 $ python3 -m pip install --user ".[gui,crypto]"
 ```
 
@@ -37,7 +37,7 @@ For elliptic curve operations,
 [libsecp256k1](https://github.com/bitcoin-core/secp256k1)
 is a required dependency:
 ```
-$ sudo apt-get install libsecp256k1-0
+$ sudo apt-get install libsecp256k1-dev
 ```
 
 Alternatively, when running from a cloned repository, a script is provided to build