Update comment to make doxygen happy

Doxygen produced a warning about a reference it couldn't find, because
of a '#' symbol used in the comment (which the author didn't intend to
be a reference).  Usually, '#' could be escaped '\#', but that doesn't
work in this case, because it appears within quotes (as explained in
https://stackoverflow.com/a/36750674 ).  I couldn't find a way to get
this to work with the quotes that had been used, so I have changed it
to use different quotes.

Author: John Nonweiler

src/util/README.md

@@ -89,7 +89,7 @@ efficient to store many copies of the same string. The static list of strings
 is initially populated from `irep_ids.def`, so for example the fourth entry
 in `irep_ids.def` is `“IREP_ID_ONE(type)”`, so the string “type” has index 3.
 You can refer to this \ref irep_idt as `ID_type`. The other kind of line you
-see is `“IREP_ID_TWO(C_source_location, #source_location)”`, which means the
+see is \c "IREP_ID_TWO(C_source_location, #source_location)", which means the
 \ref irep_idt for the string “\#source_location” can be referred to as
 `ID_C_source_location`. The “C” is for comment, meaning that it should be
 stored in the [comments](\ref irept::dt::comments). Any strings that need