runtime(doc): handle newlines in base64 string encode example

closes: #17777

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

Author: zeertzjq

runtime/doc/builtin.txt

@@ -1,4 +1,4 @@
-*builtin.txt*	For Vim version 9.1.  Last change: 2025 Jul 14
+*builtin.txt*	For Vim version 9.1.  Last change: 2025 Jul 17
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -1268,7 +1268,7 @@ base64_encode({blob})					*base64_encode()*
 		    " Encode the contents of a binary file
 		    echo base64_encode(readblob('somefile.bin'))
 		    " Encode a string
-		    echo base64_encode(str2blob([somestr]))
+		    echo base64_encode(str2blob(somestr->split("\n")))
 <
 		Can also be used as a |method|: >
 			GetBinaryData()->base64_encode()