spec: slightly rephrased wording on parsing ambiguity for composite literals
spec: slightly rephrased wording on parsing ambiguity for composite literals

Fixes issue 4482.

LGTM=r
R=r, iant, rsc, ken
CC=golang-codereviews
https://codereview.appspot.com/69020045

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang