runtime(doc): Normalise builtin-function optional parameter formatting

These should generally be formatted as func([{arg}]) and referenced as
{arg} in the description.

closes: #14438

Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/testing.txt b/runtime/doc/testing.txt
index 356d74e..9b9f60b 100644
--- a/runtime/doc/testing.txt
+++ b/runtime/doc/testing.txt
@@ -1,4 +1,4 @@
-*testing.txt*	For Vim version 9.1.  Last change: 2024 Feb 18
+*testing.txt*	For Vim version 9.1.  Last change: 2024 Apr 07
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -438,8 +438,8 @@
 			GetTime()->test_settime()
 
 
-test_srand_seed([seed])					*test_srand_seed()*
-		When [seed] is given this sets the seed value used by
+test_srand_seed([{seed}])				*test_srand_seed()*
+		When {seed} is given this sets the seed value used by
 		`srand()`.  When omitted the test seed is removed.