spelling: shallow

Signed-off-by: Josh Soref <2119212+jsoref@users.noreply.github.com>

doc/html/boost_optional/relnotes.html

@@ -220,7 +220,7 @@
                 are now turned into compile-time errors,
               </li>
 <li class="listitem">
-                you can swap optional references: it is like swapping pointers: shalow,
+                you can swap optional references: it is like swapping pointers: shallow,
                 underlying objects are not affected,
               </li>
 <li class="listitem">