fixing another typo in comment

IPython/terminal/debugger.py

@@ -88,7 +88,7 b" if __name__ == '__main__':"
     import pdb
     # IPython.core.debugger.Pdb.trace_dispatch shall not catch
     # bdb.BdbQuit. When started through __main__ and an exception
-    # happend after hitting "c", this is needed in order to
+    # happened after hitting "c", this is needed in order to
     # be able to quit the debugging session (see #9950).
     old_trace_dispatch = pdb.Pdb.trace_dispatch
     pdb.Pdb = TerminalPdb