Slightly revise cache documentation remarks

There was a typo.

Author: pkgw

docs/src/getting-started/first-document.md

@@ -135,17 +135,17 @@ Tectonic’s “user experience” is substantially different from those engines
 We hope that you’ll agree that these changes make for a program that is much
 more pleasant to use than the traditional tools.
 
+
 ## Cache
+
 The location of the cache depends on your operating system. You can use the
 [V2 Interface][v2cli-ref] to find the exact cache location on your machine
 or take a look [at the implementation][user-cache-impl].
 
-We recommend leaving the cache location at the default settings unless you
-have good reasons to change it, for example due to constrains within
-the environment tectonic is running in.
-
-If you need to change the location of the cache, you can do that by providing
-a custom cache location with the environment variable `TECTONIC_CACHE_DIR`.
+If you need to change the location of the cache, you can do that by setting
+the environment variable `TECTONIC_CACHE_DIR` to the path of a directory.
+We recommend leaving the cache location at the default unless there is a
+compelling reason to change it.
 
 [v2cli-ref]: ../ref/v2cli.md
 [user-cache-impl]: https://docs.rs/tectonic_io_base/latest/tectonic_io_base/app_dirs/fn.ensure_user_cache_dir.html