docs: correct README code for optional

Author: jamietanna

README.md

@@ -14,8 +14,9 @@ And can be embedded in structs, for instance with the following definition:
 obj := struct {
 		// RequiredID is a required, nullable field
 		RequiredID     nullable.Nullable[int]     `json:"id"`
-		// RequiredID is an optional, nullable field
-		OptionalString *nullable.Nullable[string] `json:"optionalString,omitempty"`
+		// OptionalString is an optional, nullable field
+		// NOTE that no pointer is required, only `omiempty`
+		OptionalString nullable.Nullable[string] `json:"optionalString,omitempty"`
 }{}
 ```