docs: add missing change to getcharstr() signature (#33797)

runtime/doc/builtin.txt

@@ -3317,7 +3317,7 @@ getcharsearch()                                                *getcharsearch()*
                 Return: ~
                   (`table`)
 
-getcharstr([{expr}])                                              *getcharstr()*
+getcharstr([{expr} [, {opts}]])                                   *getcharstr()*
 		The same as |getchar()|, except that this always returns a
 		String, and "number" isn't allowed in {opts}.
 

src/nvim/eval.lua

@@ -3730,7 +3730,7 @@ M.funcs = {
     name = 'getcharstr',
     params = { { 'expr', '-1|0|1' }, { 'opts', 'table' } },
     returns = 'string',
-    signature = 'getcharstr([{expr}])',
+    signature = 'getcharstr([{expr} [, {opts}]])',
   },
   getcmdcomplpat = {
     desc = [=[