ch-13: fix grammar re attack surface

Reported by @ogunden.

13_wire_protocol.asciidoc

@@ -160,10 +160,10 @@ implementation as well as the software that interacts with message parsing. A
 curious reader might ask "why not just use protobufs"? In response, the
 Lighting developers would respond that we're able to have the best of the
 extensibility of protobufs while also having the benefit of a smaller
-implementation and thus attacks surface in the context of Lightning. As of
-version v3.15.6, the protobuf compiler weighs in at over 656,671 lines of code.
-In comparison lnd's implementation of the TLV message format weighs in at only
-2.3k lines of code (including tests).
+implementation and thus smaller attack. As of version v3.15.6, the protobuf
+compiler weighs in at over 656,671 lines of code.  In comparison lnd's
+implementation of the TLV message format weighs in at only 2.3k lines of code
+(including tests).
 
 With the necessary background presented, we're now ready to describe the TLV
 format in detail. A TLV message extension is said to be a _stream_ of