docs(nix-1p): clarify wording for what is an expression

See #19

Change-Id: Ie78eb00192b595ffa59040a4f6d311023621939a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10737
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Autosubmit: tazjin <tazjin@tvl.su>

Author: tazjin

README.md

@@ -50,8 +50,7 @@ Nix is:
     any dependency between operations is established by depending on *data* from
     previous operations.
 
-    Everything in Nix is an expression, meaning that every directive returns
-    some kind of data.
+    Any valid piece of Nix code is an *expression* that returns a value.
 
     Evaluating a Nix expression *yields a single data structure*, it does not
     execute a sequence of operations.