Update definition of Unist utilities

Where previously Unist utilities were defines as working with both
nodes and virtual files, the latter is removed, thus narrowing the
definition.

In practice, those utilities are in the scope of `vfile`, not Unist.

readme.md

@@ -124,12 +124,12 @@ existing files.  Neither are they limited to the file-system only.
 
 ## Unist utilities
 
-**Unist utilities** are function which work with **unist nodes** or **unist
-files**, agnostic of **remark**, **retext**, or **hast**.
+**Unist utilities** are function which work with **unist nodes**,
+agnostic of **remark**, **retext**, or **hast**.
 
 A list of **VFile**-related utilities can be found at [**vfile**][vfile].
 
-### Unist node utilties
+### List of Utilties
 
 *   [`unist-util-filter`](https://github.com/eush77/unist-util-filter)
     — Create a new Unist tree with all nodes that pass the test