Fix markup error

Doc/whatsnew/whatsnew20.tex

@@ -280,7 +280,7 @@ by 2, equivalent to the slightly lengthier
 
 The full list of supported assignment operators is \code{+=},
 \code{-=}, \code{*=}, \code{/=}, \code{\%=}, \code{**=}, \code{\&=},
-\code{|=}, \code{^=}, \code{>>=}, and \code{<<=}.  Python classes can
+\code{|=}, \verb|^=|, \code{>>=}, and \code{<<=}.  Python classes can
 override the augmented assignment operators by defining methods named
 \method{__iadd__}, \method{__isub__}, etc.  For example, the following
 \class{Number} class stores a number and supports using += to create a