fixup: typo in name of feature-test macro

Author: jensmaurer

source/preprocessor.tex

@@ -1869,8 +1869,8 @@
 \defnxname{cpp_unicode_literals}                  & \tcode{200710L} \\ \rowsep
 \defnxname{cpp_user_defined_literals}             & \tcode{200809L} \\ \rowsep
 \defnxname{cpp_using_enum}                        & \tcode{201907L} \\ \rowsep
-\defnxname{cpp_variable_friend}                   & \tcode{202403L} \\ \rowsep
 \defnxname{cpp_variable_templates}                & \tcode{201304L} \\ \rowsep
+\defnxname{cpp_variadic_friend}                   & \tcode{202403L} \\ \rowsep
 \defnxname{cpp_variadic_templates}                & \tcode{200704L} \\ \rowsep
 \defnxname{cpp_variadic_using}                    & \tcode{201611L} \\
 \end{LongTable}