replace in opening quotations

node14.html

@@ -107,7 +107,7 @@ the smallest <IMG
  ALT="$\tau$"> if any at which a signal repeats is called the signal's
 <A NAME="1175"></A><I>period</I>.
 In discussing periods of digital audio signals, we quickly run into the
-difficulty of describing signals whose ``period" isn't an integer, so that the
+difficulty of describing signals whose "period" isn't an integer, so that the
 equation above doesn't make sense.  For now we'll  effectively
 ignore this difficulty by supposing that the signal <IMG
  WIDTH="31" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"