Fix wording (#682)

insta/src/lib.rs

@@ -68,7 +68,7 @@
 //! directly from `cargo test` and control it via the `INSTA_UPDATE` environment
 //! variable — see [Updating snapshots](#updating-snapshots) for details.
 //!
-//! You can for instance first run the tests and not write and new snapshots, and
+//! You can for instance first run the tests and not write any new snapshots, and
 //! if you like them run the tests again and update them:
 //!
 //! ```text