Update Copilot instructions to search for call chains across multiple lines

I've noticed lately that Copilot often greps for function call chains like `obj.foo().bar()` when, in fact, rustfmt might've reformatted it across multiple lines like,

```rust
obj
    .foo()
    .bar();
```

Update the copilot-instructions.md to take this into account whenever it's grep'ing for function call chains.