runtime(doc): fix base64 encode/decode examples

Use str2blob()/blob2str() functions in examples given.

closes: #16449

Signed-off-by: Maxim Kim <habamax@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/builtin.txt b/runtime/doc/builtin.txt
index 10970bc..9ba22dc 100644
--- a/runtime/doc/builtin.txt
+++ b/runtime/doc/builtin.txt
@@ -1,4 +1,4 @@
-*builtin.txt*	For Vim version 9.1.  Last change: 2025 Jan 14
+*builtin.txt*	For Vim version 9.1.  Last change: 2025 Jan 15
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -1244,7 +1244,7 @@
 		    " Write the decoded contents to a binary file
 		    call writefile(base64_decode(s), 'tools.bmp')
 		    " Decode a base64-encoded string
-		    echo list2str(blob2list(base64_decode(encodedstr)))
+		    echo blob2str(base64_decode(encodedstr))
 <
 		Can also be used as a |method|: >
 			GetEncodedString()->base64_decode()
@@ -1260,7 +1260,7 @@
 		    " Encode the contents of a binary file
 		    echo base64_encode(readblob('somefile.bin'))
 		    " Encode a string
-		    echo base64_encode(list2blob(str2list(somestr)))
+		    echo base64_encode(str2blob(somestr))
 <
 		Can also be used as a |method|: >
 			GetBinaryData()->base64_encode()