rewrite a few section titles

html/documentation.html

@@ -54,7 +54,7 @@ title: Documentation - Frama-C
 
           <div class="tileWrapper" size="small">
             <div class="tileWrapperTitle">
-              <span>Plug-ins Manuals</span>
+              <span>Plug-in Manuals</span>
             </div>
             <div class="tile">
               <h4 class="tileTitle"><span>Plugin Development</span></h4>
@@ -96,4 +96,4 @@ title: Documentation - Frama-C
       <div class="clear"></div>
     </div>
   </div>
-</body>
+</body>
\ No newline at end of file