Drop misleading ellipses

This portion of the quick start guide may lead to confusion for
newcomers to Nix.  This change clarifies the example to one that can be
copied in its entirety.

doc/manual/introduction/quick-start.xml

@@ -40,7 +40,7 @@ libxslt-1.1.28
 <step><para>Install some packages from the channel:
 
 <screen>
-$ nix-env -i hello <replaceable>...</replaceable> </screen>
+$ nix-env -i hello</screen>
 
 This should download pre-built packages; it should not build them
 locally (if it does, something went wrong).</para></step>