Fix cargo publish error due to examples without path

[mgeisler]

Without this, we get a strange error:

% cargo publish --dry-run
    Updating crates.io index
   Packaging textwrap v0.15.0 (/home/mg/src/textwrap)
   Verifying textwrap v0.15.0 (/home/mg/src/textwrap)
error: failed to verify package tarball

Caused by:
  failed to parse manifest at `/home/mg/src/textwrap/target/package/textwrap-0.15.0/Cargo.toml`

Caused by:
  can't find `hyphenation` example at `examples/hyphenation.rs` or
  `examples/hyphenation/main.rs`. Please specify example.path if you
  want to use a non-default path.

This happens even though the file is there:

% ls examples/hyphenation.rs
examples/hyphenation.rs

Specifying an explicit path helps and should let the release go
through.