Fix spelling of primitive types

Primitive types have been renamed in pull request carbon-language#861 and also the design document.

docs/design/primitive_types.md

@@ -37,14 +37,14 @@ modifying other types. They also have semantics that are defined from first
 principles rather than in terms of other operations. These will be made
 available through the [prelude package](README.md#name-lookup-for-common-types).
 
--   `Bool` - a boolean type with two possible values: `True` and `False`.
+-   `Bool` - a boolean type with two possible values: `true` and `false`.
 -   `Int` and `UInt` - signed and unsigned 64-bit integer types.
     -   Standard sizes are available, both signed and unsigned, including
-        `Int8`, `Int16`, `Int32`, `Int128`, and `Int256`.
+        `i8`, `i16`, `i32`, `i64`, `i128`, and `i256`.
     -   Overflow in either direction is an error.
 -   `Float64` - a floating point type with semantics based on IEEE-754.
-    -   Standard sizes are available, including `Float16`, `Float32`, and
-        `Float128`.
+    -   Standard sizes are available, including `f16`, `f32`, and
+        `f128`.
     -   [`BFloat16`](primitive_types.md#bfloat16) is also provided.
 -   `String` - a byte sequence treated as containing UTF-8 encoded text.
     -   `StringView` - a read-only reference to a byte sequence treated as