small change to error message

kwant/__init__.py

@@ -12,8 +12,8 @@ try:
     from . import _system
 except ImportError:
     msg = """Error importing Kwant: You should not try to import Kwant from
-    its source directory.  Please exit the Kwant source tree, and relaunch
-    your Python intepreter from there."""
+    its source directory.  Please exit the Kwant source distribution directory,
+    and relaunch your Python intepreter from there."""
     raise ImportError(msg)
 
 __all__ = ['system', 'version', 'builder', 'lattice', 'solvers',