Update manual.md (nim-lang#23393)

adding link to generic == for tuples in Open and Closed symbols example

doc/manual.md

@@ -5863,7 +5863,7 @@ at definition and the context at instantiation are considered:
   echo a == b # works!
   ```
 
-In the example, the generic `==` for tuples (as defined in the system module)
+In the example, the [generic `==` for tuples](system.html#%3D%3D%2CT%2CT_2) (as defined in the system module)
 uses the `==` operators of the tuple's components. However, the `==` for
 the `Index` type is defined *after* the `==` for tuples; yet the example
 compiles as the instantiation takes the currently defined symbols into account