[atomics.types.operations] Remove uses of deprecated ATOMIC_VAR_INIT

from examples.

Author: zygoloid

source/atomics.tex

@@ -1952,7 +1952,7 @@
   // Padding here.
   unsigned biff = 0xC0DEFEFE;
 };
-atomic<padded> pad = ATOMIC_VAR_INIT({});
+atomic<padded> pad = {};
 
 bool zap() {
   padded expected, desired{0, 0};
@@ -1971,7 +1971,7 @@
   double celestia = 0.;
   short luna;       // padded
 };
-atomic<pony> princesses = ATOMIC_VAR_INIT({});
+atomic<pony> princesses = {};
 
 bool party(pony desired) {
   pony expected;