[complex.ops] use character literal for single character

Author: jwakely

source/numerics.tex

@@ -728,7 +728,7 @@
 s.flags(o.flags());
 s.imbue(o.getloc());
 s.precision(o.precision());
-s << '(' << x.real() << "," << x.imag() << ')';
+s << '(' << x.real() << ',' << x.imag() << ')';
 return o << s.str();
 \end{codeblock}