Removed ref to spec in the instructions

The specification of teal-2 is not present.

Exercise3.md

@@ -172,7 +172,6 @@ Builtins should not return null, except `print`, but that case can be ignored.
 ## Resources
 
 - The [README.md](README.org) file contains general information about building, running, and extending Teal.
-- You can find the language specification for Teal-0 [here](docs/teal-0.pdf).
 - To test your program, you can use the Teal test programs that ship with
   the distribution, in `compiler/testfiles/interpreter/*.in`, but you may
   find it useful to build your own.  (Note that not all of the programs in that directory will pass type checking!)