[simd.mask.overview] Fix markup; add two missing closing @

Author: mattkretz

source/numerics.tex

@@ -20045,8 +20045,8 @@
 namespace std::simd {
   template<size_t Bytes, class Abi> class basic_mask {
   public:
-    using @\libmember{value_type}{basic_mask} = bool;
-    using @\libmember{abi_type}{basic_mask} = Abi;
+    using @\libmember{value_type}{basic_mask}@ = bool;
+    using @\libmember{abi_type}{basic_mask}@ = Abi;
     using @\libmember{iterator}{basic_mask}@ = @\exposid{simd-iterator}@<basic_mask>;
     using @\libmember{const_iterator}{basic_mask}@ = @\exposid{simd-iterator}@<const basic_mask>;