learn/manuals: quick link to temporal was missing icon (#429)

Font awesome icon name fixed, now it displays a clock.

themes/grass/layouts/partials/quicklinks.html

@@ -14,7 +14,7 @@ <h4 class="mt-20 tind"><i class="ms ms-data-cube-o ms-fw"></i> Raster 3d <a href
 <h4 class="mt-20 tind"><i class="ms ms-database-o ms-fw"></i> Database <a href="/grass{{- .Site.Data.grass.current_version_nodots -}}/manuals/database.html" target="_blank">(db.*)</a></h4>
 </div>
 <div class="card mb-1-5">
-<h4 class="mt-20 tind"><i class="fa fa-clock-o fa-fw"></i> Temporal <a href="/grass{{- .Site.Data.grass.current_version_nodots -}}/manuals/temporal.html" target="_blank">(t.*)</a></h4>
+<h4 class="mt-20 tind"><i class="fa fa-clock fa-fw"></i> Temporal <a href="/grass{{- .Site.Data.grass.current_version_nodots -}}/manuals/temporal.html" target="_blank">(t.*)</a></h4>
 </div>
 <div class="card mb-1-5">
 <h4 class="mt-20 tind"><i class="ms ms-map-rolled-o ms-fw"></i> Display <a href="/grass{{- .Site.Data.grass.current_version_nodots -}}/manuals/display.html" target="_blank">(d.*)</a></h4>