fix typo

[[Imported from SVN: r1522]]

COPYING

 The DUNE library and headers are licensed under version 2 of the GNU
 General Public License (see below), with a special exception for
 linking and compiling against DUNE, the so-called "runtime exception."
-The license is intended to be similiar to the GNU Lesser General
+The license is intended to be similar to the GNU Lesser General
 Public License, which by itself isn't suitable for a template library.
 
 The exact wording of the exception reads as follows: