Fix doc compile error, center pseudo headers

doc/Background.tex

@@ -394,16 +394,17 @@ important because some targets (like the NetFPGA) do not allow
 accessing the payload (see section \ref{design:netpfga}).
 \begin{figure}[h]
 \begin{verbatim}
-0        7 8     15 16    23 24    31
-+--------+--------+--------+--------+
-|          source address           |
-+--------+--------+--------+--------+
-|        destination address        |
-+--------+--------+--------+--------+
-|  zero  |protocol| TCP/UDP length  |
-+--------+--------+--------+--------+
+                0        7 8     15 16    23 24    31
+                +--------+--------+--------+--------+
+                |          source address           |
+                +--------+--------+--------+--------+
+                |        destination address        |
+                +--------+--------+--------+--------+
+                |  zero  |protocol| TCP/UDP length  |
+                +--------+--------+--------+--------+
 \end{verbatim}
-  \centering
+    \centering
+
   \caption{IPv4 Pseudo Header}
   \label{fig:ipv4pseudoheader}
 \end{figure}

doc/Introduction.tex

@@ -68,7 +68,7 @@ with legacy IPv4 devices still needs to be provided.
 \end{figure}
 IPv6 hosts and IPv4 hosts cannot directly connect to each other,
 because the protocols are incompatible to each other.
-To allow communication between different protocol host!s,
+To allow communication between different protocol hosts,
 several transition mechanisms have been
 proposed~\cite{wikipedia:_ipv6},~\cite{rfc4213}.
 \begin{figure}[h]

doc/refs/refs.bib

@@ -92,7 +92,7 @@
 @Misc{lutchansky:_tayga_simpl_nat64_linux,
   author =    {Nathan Lutchansky},
   title =     {TAYGA - Simple, no-fuss NAT64 for Linux},
-  howpublished = {\url{http://www.litech.org/tayga/}}
+  howpublished = {\url{http://www.litech.org/tayga/}},
   note =      {Requested on 2019-08-22}}