Error displays should be lowercase

[aecsocket]

Nitpick but it makes the rest of the error chain look strange.

The Rust API docs state:

The error message given by the Display representation of an error type should be lowercase without trailing punctuation, and typically concise.

Currently, this crate doesn't conform to this style, but this shouldn't be too big of a change.

I.e.

/// An enumeration representing various errors that can occur during a WebTransport client connecting.
#[derive(thiserror::Error, Debug)]
pub enum ConnectingError {
    /// URL provided for connection is not valid.
    #[error("Invalid URL: {0}")]
    InvalidUrl(String),

to

/// An enumeration representing various errors that can occur during a WebTransport client connecting.
#[derive(thiserror::Error, Debug)]
pub enum ConnectingError {
    /// URL provided for connection is not valid.
    #[error("invalid URL: {0}")]
    InvalidUrl(String),