chapter06: Remove extra character in groff page

chapter06/groff.xml

@@ -538,7 +538,7 @@
       <varlistentry id="roff2x">
         <term><command>roff2x</command></term>
         <listitem>
-          <para>将 roff 文件转换成其他格式s</para>
+          <para>将 roff 文件转换成其他格式</para>
           <indexterm zone="ch-system-groff roff2x">
             <primary sortas="b-roff2x">roff2x</primary>
           </indexterm>