Fix pub_key comment in dh.h.

(Imported from upstream's 602af7d01da82dc6c45946572f531bcbe9ebbf47.)

Change-Id: I17cc0bfaf2d9954aafae9da30d2d37a97101f330
Reviewed-on: https://boringssl-review.googlesource.com/5691
Reviewed-by: Adam Langley <agl@google.com>