runtime: fix getcallersp documentation
runtime: fix getcallersp documentation

LGTM=bradfitz
R=rsc, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/180760043

bryanxu/go-zh,d0f/go-zh,d0f/go-zh,webfd/go-zh,webfd/go-zh,d0f/go-zh,webfd/go-zh,bryanxu/go-zh,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,webfd/go-zh,webfd/go-zh,webfd/go-zh,d0f/go-zh,bryanxu/go-zh,webfd/go-zh,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,bryanxu/go-zh,d0f/go-zh,bryanxu/go-zh,d0f/go-zh