doc: fix confusion markdown in util.markdown

PR-URL: #1097
Reviewed-By: Brendan Ashworth <[email protected]>

Author: yorkie

doc/api/util.markdown

@@ -54,7 +54,7 @@ argument. Supported placeholders are:
 * `%s` - String.
 * `%d` - Number (both integer and float).
 * `%j` - JSON.  Replaced with the string `'[Circular]'` if the argument
-         contains circular references.
+contains circular references.
 * `%%` - single percent sign (`'%'`). This does not consume an argument.
 
 If the placeholder does not have a corresponding argument, the placeholder is