updated "syntax" to highlight optional arguments as optional

Co-authored-by: Jeremie Vandenplas <jeremie.vandenplas@gmail.com>

Author: aman-godara

doc/specs/stdlib_strings.md

@@ -286,7 +286,7 @@ If `occurrence`th occurrence is not found, function returns `0`.
 
 #### Syntax
 
-`string = [[stdlib_strings(module):find(interface)]] (string, pattern, occurrence, consider_overlapping)`
+`string = [[stdlib_strings(module):find(interface)]] (string, pattern [, occurrence, consider_overlapping])`
 
 #### Status
 
@@ -327,4 +327,4 @@ program demo_find
   print *, find("qwqwqwq", "qwq", 3, [.false., .true.]) ! [0, 5]
 
 end program demo_find
-```
+```