DOC: Use valid completion in docs for unicode completions.

The unicode names are all upper case, so the example in the docstring doesn't
actually work.

IPython/core/completer.py

@@ -35,7 +35,7 b' or using unicode completion:'
 
 .. code::
 
-    \\greek small letter alpha<tab>
+    \\GREEK SMALL LETTER ALPHA<tab>
     α