Link 'modern generics system', not just 'generics'. (#1317)

Tiny nit: Line 63 similarly links "modern generics system". Line 250 links 'generics', but doesn't include "modern ... system." So I think this approach is consistent (and subtly, implies the link is about the modern ... system, not just "generics").

README.md

@@ -201,9 +201,9 @@ code to help you switch an existing C++ codebase to Carbon.
 
 ## Generics
 
-Carbon provides a **modern
-[generics](/docs/design/generics/overview.md#what-are-generics) system** with
-checked definitions, while still **supporting opt-in
+Carbon provides a
+**[modern generics system](/docs/design/generics/overview.md#what-are-generics)**
+with checked definitions, while still **supporting opt-in
 [templates](/docs/design/templates.md) for seamless C++ interop**. Checked
 generics provide several advantages compared to C++ templates: