runtime(doc): wrong return type for execute() function

The description clearly explains that this function returns a string.  I
think the 'Number' here is a copy/paste error.

closes: #15045

Signed-off-by: Marius Gedminas <marius@gedmin.as>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/builtin.txt b/runtime/doc/builtin.txt
index 0061e9e..c1dd260 100644
--- a/runtime/doc/builtin.txt
+++ b/runtime/doc/builtin.txt
@@ -1,4 +1,4 @@
-*builtin.txt*	For Vim version 9.1.  Last change: 2024 Jun 17
+*builtin.txt*	For Vim version 9.1.  Last change: 2024 Jun 19
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -2580,7 +2580,7 @@
 		Can also be used as a |method|: >
 			GetCommand()->execute()
 <
-		Return type: |Number|
+		Return type: |String|
 
 
 exepath({expr})						*exepath()*