Incomplete sentence in _`let` bindings_ (#1697)

The original sentence was grammatically unsound, specified the three options of copy, move or pointer binding in parenthesis and some words around it.

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

docs/design/README.md

@@ -698,10 +698,11 @@ A `let`-binding may trigger a copy of the original value, or a move if the
 original value is a temporary, or the binding may be a pointer to the original
 value, like a
 [`const` reference in C++](<https://en.wikipedia.org/wiki/Reference_(C%2B%2B)>).
-Which option must not be observable to the programmer. For example, Carbon will
-not allow modifications to the original value when it is through a pointer. This
-choice may also be influenced by the type. For example, types that don't support
-being copied will be passed by pointer instead.
+Which of these options (copy, move, or pointer) is selected must not be
+observable to the programmer. For example, Carbon will not allow modifications
+to the original value when it is through a pointer. This choice may also be
+influenced by the type. For example, types that don't support being copied will
+be passed by pointer instead.
 
 A [generic binding](#checked-and-template-parameters) uses `:!` instead of a
 colon (`:`) and can only match