doc: Rewrite opening paragraph of CFI Design Doc

[1c3t3a]

Hi @pcc and @sunshaoce, I picked you as reviewers since you recently committed to the file. Please feel free to re-route this patch to the correct reviewer :)

[sunshaoce]

I tend to keep it as it is.

clang/docs/ControlFlowIntegrityDesign.rst

 ===========================================
 Control Flow Integrity Design Documentation
 ===========================================
 
 This page documents the design of the :doc:`ControlFlowIntegrity` schemes
 supported by Clang.
 
 Forward-Edge CFI for Virtual Calls
 ==================================
 
-This scheme works by allocating, for each static type used to make a virtual
-call, a region of read-only storage in the object file holding a bit vector
-that maps onto to the region of storage used for those virtual tables. Each
-set bit in the bit vector corresponds to the `address point`_ for a virtual
-table compatible with the static type for which the bit vector is being built.
+This scheme works by allocating a region of read-only storage in the object
+file that holds a bit vector which maps onto the region of storage used for
+the virtual tables. This is done for each static type that is used to make a
+virtual call. Each set bit in the bit vector corresponds to the `address point`_
+for a virtual table compatible with the static type for which the bit vector is
+being built.
 
 For example, consider the following three C++ classes:
 
 .. code-block:: c++
 
   struct A {
     virtual void f1();
     virtual void f2();