Fix build error on Rust 1.56
Fix build error on Rust 1.56

Something changed between Rust 1.57 and 1.58 so that the slice pattern
doesn’t work without an explicit type annotation in the earlier
version. Without it, we get

```
error[E0277]: expected a `Fn<(char,)>` closure, found `[char; 3]`
   --> src/lib.rs:350:52
    |
350 |             let mut needs_cap = sentence.ends_with(punctuation);
    |                                                    ^^^^^^^^^^^ expected an `Fn<(char,)>` closure, found `[char; 3]`
    |
    = help: the trait `Fn<(char,)>` is not implemented for `[char; 3]`
    = note: required because of the requirements on the impl of `FnOnce<(char,)>` for `&[char; 3]`
    = note: required because of the requirements on the impl of `Pattern<'_>` for `&[char; 3]`
```

Between 1.57 and 1.58, the `Pattern` trait got a new implementation
for `[char; N]`, and I guess this somehow made the code work
afterwards (even though we’re using a `[char]` here).

The problematic code change was introduced in #75.

Fixes #83.

mgeisler/lipsum