Fix trypo

Author: itziakos

chaco/downsample/lttb.py

@@ -8,7 +8,7 @@
 #
 # Thanks for using Enthought open source!
 import logging
-import platofrm
+import platform
 
 logger = logging.getLogger(__name__)
 
@@ -20,7 +20,7 @@
     logger.warning(
         "Can't import _lttb extension module, lttb downsampling will not work."
     )
-if platofrm.system() == 'Darwin':
+if platform.system() == 'Darwin':
     #  Disabling acceleration on MacOS see
     # https://github.com/enthought/chaco/issues/918
     _lttb = None