Replace null with "null" in YAML examples

YAML coerces `null` to a null literal (None in Python) rather than a string
literal ("null"). The examples should say `"null"` since all type attribues in
Recap have a string literal value.

specs/type/0.2.0.md

@@ -268,7 +268,7 @@ A value that can be one of several types. It is acceptable for a value to be mor
 # A union type of null or a 32-bit signed int
 type: union
 types:
-  - type: null
+  - type: "null"
   - type: int
     bits: 32
 ```
@@ -292,7 +292,7 @@ All types support a `doc` attribute, which allows developers to document types.
 type: union
 doc: A union type of null or a 32-bit signed int
 types:
-    - type: null
+    - type: "null"
     - type: int
       bits: 32
 ```
@@ -330,7 +330,7 @@ fields:
   - name: secondary_phone
     type: union
     types: ["null", "string32"]
-    default: null
+    default: "null"
 ```
 
 Unions can be cumbersome, so Recap supports a shorthand syntax for types using `optional`: