Correct keyword arg name for print builtin to sep

[ben-krieger]

This fixes a bug where the intrinsic argument for the print builtin separator was incorrectly named "str" instead of "sep". The fix changes a single string literal to match the actual print(..., sep=...) keyword argument.

Before the fix:

and

But using "str" obviously failed on run:

$ starlark hello.star
Traceback (most recent call last):
  hello.star:4:5: in <toplevel>
  hello.star:2:10: in main
Error in print: print: unexpected keyword argument "str"