Update Nginx rules in installation guide

documentation/installation_guide.html

@@ -690,6 +690,7 @@ If you have issues with files taking a long time to download, make sure you reco
   location / {
     rewrite ^/courses/([^/]+)/$ /main/course_home/course_home.php?cDir=$1 last;
     rewrite ^/courses/([^/]+)/index.php$ main/course_home/course_home.php?cDir=$1 last;
+    rewrite ^/main/admin/$ /main/admin/index.php last;
     try_files $uri @rewrite;
   }
 

documentation/installation_guide_es_ES.html

@@ -691,6 +691,7 @@ por ejemplo. El efecto debería ser inmediato.
   location / {
     rewrite ^/courses/([^/]+)/$ /main/course_home/course_home.php?cDir=$1 last;
     rewrite ^/courses/([^/]+)/index.php$ main/course_home/course_home.php?cDir=$1 last;
+    rewrite ^/main/admin/$ /main/admin/index.php last;
     try_files $uri @rewrite;
   }
 

documentation/installation_guide_fr_FR.html

@@ -757,6 +757,7 @@ Ce sont uniquement les redirections à placer dans un bloc server{}, comme les a
   location / {
     rewrite ^/courses/([^/]+)/$ /main/course_home/course_home.php?cDir=$1 last;
     rewrite ^/courses/([^/]+)/index.php$ main/course_home/course_home.php?cDir=$1 last;
+    rewrite ^/main/admin/$ /main/admin/index.php last;
     try_files $uri @rewrite;
   }