[bugfix,documentation] Fix misspelled comments

dune/common/parallel/indexset.hh

@@ -412,7 +412,7 @@ namespace Dune
     /**
      * @brief Find the index pair with a specific global id.
      *
-     * This starts a binary search for the entry and therefor has complexity
+     * This starts a binary search for the entry and therefore has complexity
      * N log(N).
      * @param global The globally unique id of the pair.
      * @return The pair of indices for the id.
@@ -450,7 +450,7 @@ namespace Dune
      *
      * After this function returns the indices are
      * consecutively numbered beginning from 0. Let
-     * $(g_i,l_i)$, $(g_j,l_j)$ be two arbituary index
+     * $(g_i,l_i)$, $(g_j,l_j)$ be two arbitrary index
      * pairs with $g_i<g_j$ then after renumbering
      * $l_i<l_j$ will hold.
      */