ipapp: Change the docstring for `-i`.

The old one suggested it was possible to run `ipython -i. foo.py` (note
the dot after the i) which does not actually work.

IPython/terminal/ipapp.py

@@ -174,7 +174,7 b" frontend_flags['quick']=("
 frontend_flags['i'] = (
     {'TerminalIPythonApp' : {'force_interact' : True}},
     """If running code from the command line, become interactive afterwards.
-    Note: can also be given simply as '-i.'"""
+    Note: can also be given simply as '-i'."""
 )
 flags.update(frontend_flags)