Martin von Zweigbergk <martinvonz@google.com> [Wed, 10 Jun 2015 14:49:27 -0700] rev 25507
bundle2: clarify in docstring that header size is for a single header
The docstring for the header size field currently says "The total
number of Bytes used by the part headers", but the size is about a
single header, so let's change it to "header".