Add note about using nix show-derivation instead

removing broken link for writing expressions.

Replacing with nix.dev language basics link.
(not a 100% substitute)

pills/02-install-on-your-running.xml

@@ -192,9 +192,9 @@
     parse="text" /></screen>
 
     <para>
-      <link
-          xlink:href="https://nixos.org/manual/nix/stable/expressions/writing-nix-expressions.html">Nix
-      expressions</link> are used to describe packages and how to
+      Nix expressions are written in the nix language  <link
+          xlink:href="https://nix.dev/tutorials/nix-language">nix 
+      language</link> and used to describe packages and how to
       build them. <link
       xlink:href="https://nixos.org/nixpkgs/">Nixpkgs</link> is the
       repository containing all of the expressions: <link

pills/06-our-first-derivation.xml

@@ -127,8 +127,11 @@
     better to pretty print it:
   </para>
 
-  <xi:include href="./06/show-derivation.xml" />
+  <note><para>
+      If your version of nix doesn't have <literal>nix derivation show</literal>, use <literal>nix show-derivation</literal> instead.
+  </para></note>
 
+  <xi:include href="./06/show-derivation.xml" />
 
   <para>
     Ok, we can see there's an out path, but it does not exist yet. We never told