Fix links to manual.html in the system module documentation (#8467)

The HTML anchor IDs were changed, which made those links not link
to the appropriate section anymore.

lib/system.nim

@@ -1465,8 +1465,8 @@ when defined(nimdoc):
     ##
     ## Note that this is a *runtime* call and using ``quit`` inside a macro won't
     ## have any compile time effect. If you need to stop the compiler inside a
-    ## macro, use the `error <manual.html#error-pragma>`_ or `fatal
-    ## <manual.html#fatal-pragma>`_ pragmas.
+    ## macro, use the `error <manual.html#pragmas-error-pragma>`_ or `fatal
+    ## <manual.html#pragmas-fatal-pragma>`_ pragmas.
 
 elif defined(genode):
   include genode/env
@@ -2429,7 +2429,7 @@ iterator fieldPairs*[T: tuple|object](x: T): RootObj {.
   ## When you iterate over objects with different field types you have to use
   ## the compile time ``when`` instead of a runtime ``if`` to select the code
   ## you want to run for each type. To perform the comparison use the `is
-  ## operator <manual.html#is-operator>`_. Example:
+  ## operator <manual.html#generics-is-operator>`_. Example:
   ##
   ## .. code-block:: Nim
   ##