Fix %reset default reply to match message

'y/[n]' prompt indicates that 'n' is default, but it was not specified as such,
so no input resulted in asking again.

IPython/core/magic.py

@@ -983,7 +983,7 b' Currently the magic system has the following functions:\\n"""'
             ans = True
         else:
             ans = self.shell.ask_yes_no(
-                "Once deleted, variables cannot be recovered. Proceed (y/[n])? ")
+                "Once deleted, variables cannot be recovered. Proceed (y/[n])? ", default='n')
         if not ans:
             print 'Nothing done.'
             return