Fix typeface.

Author: jwakely

source/diagnostics.tex

@@ -1599,7 +1599,7 @@
 \pnum
 \postconditions \tcode{code() == ec}.
 
-string(what()).find(what_arg) != string::npos.
+\tcode{string(what()).find(what_arg) != string::npos}.
 \end{itemdescr}
 
 \indexlibrary{\idxcode{system_error}!\idxcode{system_error}}
@@ -1663,7 +1663,7 @@
 \pnum
 \postconditions \tcode{code() == error_code(ev, ecat)}.
 
-string(what()).find(what_arg) != string::npos.
+\tcode{string(what()).find(what_arg) != string::npos}.
 \end{itemdescr}
 
 \indexlibrary{\idxcode{system_error}!\idxcode{system_error}}