Fix representation of ^= operator in __ixor__() documentation.

Closes SF bug #776181.  Should be backported.

Doc/ref/ref3.tex

@@ -1836,7 +1836,7 @@ complicated).
 These methods are called to implement the augmented arithmetic
 operations (\code{+=}, \code{-=}, \code{*=}, \code{/=}, \code{\%=},
 \code{**=}, \code{<}\code{<=}, \code{>}\code{>=}, \code{\&=},
-\code{\^=}, \code{|=}).  These methods should attempt to do the
+\code{\textasciicircum=}, \code{|=}).  These methods should attempt to do the
 operation in-place (modifying \var{self}) and return the result (which
 could be, but does not have to be, \var{self}).  If a specific method
 is not defined, the augmented operation falls back to the normal