net/http: add comment to clarify whether Dir is '/' or '\'.

LGTM=bradfitz
R=bradfitz, alex.brainman
CC=golang-codereviews
https://codereview.appspot.com/168600044
doc: fix small typo in doc

LGTM=adg
R=golang-codereviews, adg
CC=golang-codereviews
https://codereview.appspot.com/170660043

Committer: Andrew Gerrand <adg@golang.org>
C: add Nick Cooper (Google CLA).

LGTM=dsymonds
R=dsymonds
CC=golang-codereviews, nmvc
https://codereview.appspot.com/169580043

Committer: David Symonds <dsymonds@golang.org>