Do trivial cleanup to module level docs

Make the module level docs uniform across the codebase (style copied
from rust-bitcoin).

src/ecdsa/mod.rs

@@ -1,4 +1,5 @@
 //! Structs and functionality related to the ECDSA signature algorithm.
+//!
 
 #[cfg(feature = "recovery")]
 mod recovery;

src/scalar.rs

@@ -4,6 +4,7 @@
 //! points. The most common type of scalars are private keys. However not all scalars are private
 //! keys. They can even be public *values*. To make handling them safer and easier this module
 //! provides the `Scalar` type and related.
+//!
 
 use core::fmt;
 

src/schnorr.rs

@@ -1,4 +1,3 @@
-//! # schnorrsig
 //! Support for Schnorr signatures.
 //!