Correct type of dir function

It should return a string, not a list.

Bug: 227245326
Test: go test
Change-Id: If0cb382dede52200c0595ae939a8064641216af1
diff --git a/mk2rbc/mk2rbc.go b/mk2rbc/mk2rbc.go
index c881751..950a1e5 100644
--- a/mk2rbc/mk2rbc.go
+++ b/mk2rbc/mk2rbc.go
@@ -77,7 +77,7 @@
 	"addprefix":                            &simpleCallParser{name: baseName + ".addprefix", returnType: starlarkTypeList},
 	"addsuffix":                            &simpleCallParser{name: baseName + ".addsuffix", returnType: starlarkTypeList},
 	"copy-files":                           &simpleCallParser{name: baseName + ".copy_files", returnType: starlarkTypeList},
-	"dir":                                  &simpleCallParser{name: baseName + ".dir", returnType: starlarkTypeList},
+	"dir":                                  &simpleCallParser{name: baseName + ".dir", returnType: starlarkTypeString},
 	"dist-for-goals":                       &simpleCallParser{name: baseName + ".mkdist_for_goals", returnType: starlarkTypeVoid, addGlobals: true},
 	"enforce-product-packages-exist":       &simpleCallParser{name: baseName + ".enforce_product_packages_exist", returnType: starlarkTypeVoid},
 	"error":                                &makeControlFuncParser{name: baseName + ".mkerror"},