runtime(doc): Add a missing '<' to the help of strutf16len() (#13168)

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/builtin.txt b/runtime/doc/builtin.txt
index a52a3df..74e024d 100644
--- a/runtime/doc/builtin.txt
+++ b/runtime/doc/builtin.txt
@@ -9443,7 +9443,7 @@
 		    echo strutf16len('😊')		returns 2
 		    echo strutf16len('ą́')		returns 1
 		    echo strutf16len('ą́', v:true)	returns 3
-
+<
 		Can also be used as a |method|: >
 			GetText()->strutf16len()
 <