doc: typo fixes

doc/configuration.xml

@@ -132,7 +132,7 @@
   </itemizedlist>
 
   <para>
-   The difference between an a package being unsupported on some system and
+   The difference between a package being unsupported on some system and
    being broken is admittedly a bit fuzzy. If a program
    <emphasis>ought</emphasis> to work on a certain platform, but doesn't, the
    platform should be included in <literal>meta.platforms</literal>, but marked
@@ -175,11 +175,12 @@
 </programlisting>
     </para>
     <para>
-     A more useful example, the following configuration allows only allows
-     flash player and visual studio code:
+     For a more useful example, try the following. This configuration
+     only allows unfree packages named flash player and visual studio
+     code:
 <programlisting>
 {
-  allowUnfreePredicate = (pkg: elem (builtins.parseDrvName pkg.name).name [ "flashplayer" "vscode" ]);
+  allowUnfreePredicate = (pkg: builtins.elem (builtins.parseDrvName pkg.name).name [ "flashplayer" "vscode" ]);
 }
 </programlisting>
     </para>
@@ -286,8 +287,8 @@
 
   <para>
    You can define a function called <varname>packageOverrides</varname> in your
-   local <filename>~/.config/nixpkgs/config.nix</filename> to override nix
-   packages. It must be a function that takes pkgs as an argument and return
+   local <filename>~/.config/nixpkgs/config.nix</filename> to override Nix
+   packages. It must be a function that takes pkgs as an argument and returns a
    modified set of packages.
 <programlisting>
 {