Typo in error message

[maryaB-osr]

I don't have permission to fix this myself, but there's a typo in turtlesim/src/turtle_frame.cpp line 119

"A turtled named [%s] already exists"

Needs to be turtle. This is an error message that will be seen by people going through the turtlesim tutorial, so it needs to be fixed.

[dirk-thomas]

Fixed in #64.