docs: SC/21---Make a couple of URLs appear in \texttt.

(cherry picked from commit 8875d3f)

docs/sc/021/sc21.tex

@@ -780,7 +780,7 @@ \section{\xlabel{data_files}Raw SCUBA-2 Data}
 When you access you data either at the JCMT or by downloading from the
 \htmladdnormallink{Science Archive}{http://www3.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/jcmt/}
 \begin{latexonly}
-\footnote{http://www3.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/jcmt/}
+\footnote{\texttt{http://www3.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/jcmt/}}
 \end{latexonly}
 you will get \emph{all} of the files listed above. Later when you
 reduce your data using the map-maker you will include \emph{all} of
@@ -1083,7 +1083,7 @@ \subsubsection{\xlabel{display_cube}Displaying time-series data}
 colour scheme -- both are controlled by buttons on the sidebar.
 
 See the \xref{\textsc{Gaia} manual}{sun214}{} for full
-details.\latex{\footnote{http://docs.jach.hawaii.edu/star/sun214.htx/sun214.html}}
+details.\latex{\footnote{\texttt{http://docs.jach.hawaii.edu/star/sun214.htx/sun214.html}}}
 
 \clearpage
 \subsection{\xlabel{regrid_map}Regridding data into a map}