strings: Update Trim example.
strings: Update Trim example.

Use an input which better shows that behaviour of the function. Only leading
and trailing runes are trimed, not intermediate ones.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/42390043

Committer: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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