Update examples/step-4/doc/results.dox

Co-authored-by: Martin Kronbichler <martin.kronbichler@uni-a.de>

examples/step-4/doc/results.dox

@@ -416,7 +416,7 @@ two correct digits of the "true" answer.
   gradients reduces the convergence rates by one order, and the move
   from domain to boundary by another half order. Here, then, each
   refinement step reduces the error not by a factor of 4 any more,
-  by only by a factor of $\sqrt{2} \approx 1.4$. It take a lot
+  by only by a factor of $\sqrt{2} \approx 1.4$. It takes a lot
   of global refinement steps to reduce the error by, say, a factor
   ten or hundred, and this is reflected in the very slow convergence
   evidenced by the table.