Fix typos and errors in constexprmem.cc

chapters/chapter_03/constexprmem.cc

+struct Example {
+    int val = -1;
+    constexpr Example(int i)
+        : val { i }
+    {
+    }
+    constexpr Example(const Example&) = default;
+    constexpr ~Example() = default;
+    constexpr auto operator=(const Example&) -> Example& = default;
+    constexpr auto get() const -> int { return val; }
+    constexpr void set(int j) { val = j; }
+};
+
+auto main() -> int
+{
+    constexpr Example ex { 50 };
+    // ex.set(42);
+    // set() can't work because ex is a constexpr variable
+    // constexpr variables are automatically const
+    static_assert(ex.get() == 0);
+}