Style of help text headings in `cargo-add` and `cargo-rm` should be consistent with clap v4

[weihanglo]

Problem

• Headings are now Title case

Ref: https://github.com/clap-rs/clap/blob/bd5a6eab3bb4c8e3bd9f75be83821f316dde8ac8/CHANGELOG.md?plain=1#L178

Steps

$ cargo rm --help
Remove dependencies from a Cargo.toml manifest file

Usage: 
    …
    …

SECTION: <---- Should be "Title case"
    …

$ cargo add --help
Add dependencies to a Cargo.toml manifest file

Usage:
  …
  …

SOURCE: <---- Should be "Title case"
  …

SECTION: <---- Should be "Title case"
  …

Possible Solution(s)

Search next_help_heading and update arguments accordingly to Title case.

Notes

No response

Version

commit b332991a57c9d055f1864de1eed93e2178d49440