[expr.pre] Change note to use grammatical English (#3601)

Author: danra

source/expressions.tex

@@ -65,7 +65,7 @@
 \begin{note}
 \indextext{overflow}%
 Treatment of division by zero, forming a remainder using a zero divisor,
-and all floating-point exceptions vary among machines, and is sometimes
+and all floating-point exceptions varies among machines, and is sometimes
 adjustable by a library function.
 \end{note}