fix a typo in the documentation of how to use string fields

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@32161 65c4cc65-6c06-0410-ace0-fbb531ad65f3

include/asterisk/stringfields.h

@@ -47,7 +47,7 @@
   
   sample = calloc(1, sizeof(*sample));
   if (sample) {
-	  if (!ast_string_field_init(sample)) {
+	  if (!ast_string_field_init(sample, 256)) {
 		  free(sample);
 		  sample = NULL;
 	  }