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".

mercurial/bundle2.py

@@ -66,7 +66,7 b' Binary format is as follow'
 
 :header size: int32
 
-  The total number of Bytes used by the part headers. When the header is empty
+  The total number of Bytes used by the part header. When the header is empty
   (size = 0) this is interpreted as the end of stream marker.
 
 :header: