fix typos

lectures/costsurfaces.html

@@ -384,15 +384,15 @@ <h3>Least cost path applications</h3>
   <li class="fragment">design optimal trails
   <li class="fragment">extract foredune ridge from coastal DEM
   <li class="fragment">find safe access routes during natural disaters (earthquakes, floods)
-  <li class="fragment">estimate time for watewater to get to treatment plant from any point in sewershed
+  <li class="fragment">estimate time for wastewater to get to treatment plant from any point in sewershed
 </ul>
 </section>
 
 <section>
    <h3>Cumulative cost surface for monitoring</h3>
 <ul>
   <li>Estimate time wastewater travels from source to wastewater plant
-  <li>Time ito traverse a grid cell is computed using flow velocity estimated by Mannings equation as function of roughness, flow depth and slope
+  <li>Time to traverse a grid cell is computed using flow velocity estimated by Mannings equation as function of roughness, flow depth and slope
   <li>Important for interpeting measured concentrations of SARS-CoV2 in wastewater to
 monitor trends in COVID-19
 </ul>