Fix FlatMap example for VectorOf

PiperOrigin-RevId: 722730263

doc/domains-reference.md

diff --git a/doc/domains-reference.md b/doc/domains-reference.md
index 7384bc8..e9977b7 100644
--- a/doc/domains-reference.md
+++ b/doc/domains-reference.md
@@ -1014,8 +1014,8 @@
 ```
 
 If `AnyVectorOfFixedLengthStrings()` had been passed to `Map()`, it would have
-generated a `Domain<Domain<std::string>>`. `FlatMap()` "flattens" this to a
-`Domain<std::string>`. Thus the name FlatMap.
+generated a `Domain<Domain<std::vector<std::string>>>`. `FlatMap()` "flattens"
+this to a `Domain<std::vector<std::string>>`. Thus the name FlatMap.
 
 Another example is a pair of numbers `(a, b)`, where `b > 2 * a`: