Refine regexp for front-matter date extraction

This should solve the bug reported by Sven Seebeck where dates in the
org-timestamp style would lose their closing bracket after a rename
operation altered their front matter.

See issue 19 on the GitHub mirror:
<https://github.com/protesilaos/denote/issues/19>.

denote-retrieve.el

@@ -43,7 +43,7 @@ The match that needs to be extracted is explicityly marked as
 group 1.")
 
 (defconst denote-retrieve--date-front-matter-regexp
-  "^\\(?:#\\+\\)?\\(?:date\\)\\s-*[:=]\\s-*[\"']?\\(?1:.*\\b\\)[\"']?"
+  "^\\(?:#\\+\\)?\\(?:date\\)\\s-*[:=]\\s-*[\"']?\\(?1:.*\\b]?\\)[\"']?"
   "Regular expression for date key and value.
 The match that needs to be extracted is explicityly marked as
 group 1.")