removed typo by replacing obseve() with observe(), fixing issue #914 (#915)

As this fixes a typo clearly visible in the source code, I have not
added any code to reproduce the error nor a unit test. Hope this is
still fine (I'm short on time currently, sorry!).

See [Issue 914](#914) and/or
the original pull request #590 that introduced this bug.

Co-authored-by: Ioannis Tziakos <[email protected]>

Author: markasbach

chaco/base_2d_plot.py

@@ -340,9 +340,9 @@ def _origin_changed(self):
 
     def _index_changed(self, old, new):
         if old is not None:
-            old.obseve(self._update_index_data, "data_changed", remove=True)
+            old.observe(self._update_index_data, "data_changed", remove=True)
         if new is not None:
-            new.obseve(self._update_index_data, "data_changed")
+            new.observe(self._update_index_data, "data_changed")
         self._update_index_data()
 
     def _value_changed(self, old, new):