internal/stringset: corrected typo in comment
internal/stringset: corrected typo in comment

Change-Id: Iee0c2e501ae2128773ecc2fe181b8e33a96e74c6
Reviewed-on: https://go-review.googlesource.com/19260
Reviewed-by: Nigel Tao <58ae08b3356928ff9c67562b4719487d54fe8348@golang.org>

cmars/tools,golang/text,cmars/tools,cmars/tools,cmars/tools