[Gardening] Fixed sigil.

Author: nate-chandler

docs/ABI/Mangling.rst

@@ -212,7 +212,7 @@ types where the metadata itself has unknown layout.)
   global ::= global 'TD'                 // dynamic dispatch thunk
   global ::= global 'Td'                 // direct method reference thunk
   global ::= global 'TI'                 // implementation of a dynamic_replaceable function
-  global :== global 'Tu'                 // async function pointer of a function
+  global ::= global 'Tu'                 // async function pointer of a function
   global ::= global 'TX'                 // function pointer of a dynamic_replaceable function
   global ::= entity entity 'TV'          // vtable override thunk, derived followed by base
   global ::= type label-list? 'D'        // type mangling for the debugger with label list for function types.