Generated field doc comments render as ///Field without space

[iainmcgin]

Three format!("Field ...") call sites in the codegen produce
#[doc = "Field ..."] attributes, which prettyplease renders verbatim
as ///Field ... - no space after the slashes:

// from generated output:
///Field 1: `sentence`
pub sentence: &'a str,

Standard rustdoc style is /// Field 1: ... with a space.

Why

prettyplease emits ///X for #[doc = "X"] verbatim - no space
insertion (attr.rs:35-38).
rustfmt doesn't fix it either (roundtrips doc attrs as-is). To get
/// X the doc string itself needs a leading space.

Fix

Three sites, one-character change each:

• buffa-codegen/src/message.rs:1057 - format!("Field ...") ->
  format!(" Field ...")
• buffa-codegen/src/view.rs:312 - same
• buffa-codegen/src/view.rs:322 - same

For reference, connectrpc-codegen handles this with a doc_attrs()
helper that prefixes each non-blank line with a space before emitting
the #[doc = ...] attribute - worth considering if more doc-string
sites get added.