subscriber: add must_use for CollectorBuilder::finish (#2271)

## Motivation

Follow up to #2239.

Discovered that

```rust
    tracing_subscriber::fmt().with_ansi(false).finish();
    tracing::info!("hello");
```
doesn't actually do anything, because `finish` returns a `Collector` and
`init` is the thing that actually installs the collector.

## Solution

Added `#[must_use]` which suggests using `try_init` instead.

tracing-subscriber/src/fmt/mod.rs

@@ -476,6 +476,7 @@ where
         subscribe::Subscribe<Registry> + Send + Sync + 'static,
 {
     /// Finish the builder, returning a new `FmtCollector`.
+    #[must_use = "you may want to use `try_init` or similar to actually install the collector."]
     pub fn finish(self) -> Collector<N, E, F, W> {
         let collector = self.inner.with_collector(Registry::default());
         Collector {