help: update copyright

Author: raveit65

help/C/index.docbook

@@ -19,7 +19,7 @@
      </abstract>
 
     <copyright xml:lang="en">
-      <year>2015</year>
+      <year>2019</year>
       <holder>MATE Documentation Project</holder>
     </copyright>
     <copyright>
@@ -1625,4 +1625,4 @@
 	   </varlistentry>
      </variablelist>	
   </section>
-</article>
+</article>