doc: Use prompt more often

doc/languages-frameworks/beam.xml

@@ -131,8 +131,8 @@
    in <literal>beamPackages</literal>, use the following command:
   </para>
 
-<programlisting>
-$ nix-env -f &quot;&lt;nixpkgs&gt;&quot; -qaP -A beamPackages
+<screen>
+<prompt>$ </prompt>nix-env -f &quot;&lt;nixpkgs&gt;&quot; -qaP -A beamPackages
 beamPackages.esqlite    esqlite-0.2.1
 beamPackages.goldrush   goldrush-0.1.7
 beamPackages.ibrowse    ibrowse-4.2.2
@@ -140,16 +140,16 @@ beamPackages.jiffy      jiffy-0.14.5
 beamPackages.lager      lager-3.0.2
 beamPackages.meck       meck-0.8.3
 beamPackages.rebar3-pc  pc-1.1.0
-</programlisting>
+</screen>
 
   <para>
    To install any of those packages into your profile, refer to them by their
    attribute path (first column):
   </para>
 
-<programlisting>
-$ nix-env -f &quot;&lt;nixpkgs&gt;&quot; -iA beamPackages.ibrowse
-</programlisting>
+<screen>
+<prompt>$ </prompt>nix-env -f &quot;&lt;nixpkgs&gt;&quot; -iA beamPackages.ibrowse
+</screen>
 
   <para>
    The attribute path of any BEAM package corresponds to the name of that

doc/languages-frameworks/perl.xml

@@ -47,13 +47,13 @@ foo = import ../path/to/foo.nix {
   in <filename>all-packages.nix</filename>. You can test building a Perl
   package as follows:
 <screen>
-$ nix-build -A perlPackages.ClassC3
+<prompt>$ </prompt>nix-build -A perlPackages.ClassC3
 </screen>
   <varname>buildPerlPackage</varname> adds <literal>perl-</literal> to the
   start of the name attribute, so the package above is actually called
   <literal>perl-Class-C3-0.21</literal>. So to install it, you can say:
 <screen>
-$ nix-env -i perl-Class-C3
+<prompt>$ </prompt>nix-env -i perl-Class-C3
 </screen>
   (Of course you can also install using the attribute name: <literal>nix-env -i
   -A perlPackages.ClassC3</literal>.)
@@ -148,7 +148,7 @@ ClassC3Componentised = buildPerlPackage rec {
   </para>
 
 <screen>
-$ nix-env -i nix-generate-from-cpan
+<prompt>$ </prompt>nix-env -i nix-generate-from-cpan
 </screen>
 
   <para>
@@ -156,7 +156,7 @@ $ nix-env -i nix-generate-from-cpan
    unpacks the corresponding package, and prints a Nix expression on standard
    output. For example:
 <screen>
-$ nix-generate-from-cpan XML::Simple
+<prompt>$ </prompt>nix-generate-from-cpan XML::Simple
   XMLSimple = buildPerlPackage rec {
     name = "XML-Simple-2.22";
     src = fetchurl {