libindex: fix collation of 'operator^='.

Author: zygoloid

source/atomics.tex

@@ -1089,7 +1089,7 @@
 \indexlibrarymember{operator-=}{atomic<\placeholder{integral}>}%
 \indexlibrarymember{operator\&=}{atomic<\placeholder{integral}>}%
 \indexlibrarymember{operator"|=}{atomic<\placeholder{integral}>}%
-\indexlibrarymember{operator\^{}=}{atomic<\placeholder{integral}>}%
+\indexlibrarymember{operator\caret=}{atomic<\placeholder{integral}>}%
 \begin{itemdecl}
 T operator @\placeholder{op}@=(T operand) volatile noexcept;
 T operator @\placeholder{op}@=(T operand) noexcept;