[docs] [issue28661] Fix code example in Python 3.5 telnetlib documentation

[R. David Murray]

R. David Murray added the comment:

Well, the example code is correct for a typical telnet service running on a unix variant.  That will output a space after the colon, so that the user's input is separated from the colon when they start to type.

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nosy: +r.david.murray

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<http://bugs.python.org/issue28661>
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