Fix TOC in readme

Author: renanbr

README.md

@@ -29,9 +29,9 @@ You are browsing the documentation of **BibTeX Parser 2.x**, the latest version.
    * [Authors and editors](#authors-and-editors)
    * [Keywords](#keywords)
    * [Date](#date)
-   * [Fill missing](#fill-missing)
-   * [Trim](#trim)
-   * [Url from DOI](#url-from-doi)
+   * [Fill missing tag](#fill-missing-tag)
+   * [Trim tags](#trim-tags)
+   * [Determine URL from the DOI](#determine-url-from-the-doi)
    * [LaTeX to unicode](#latex-to-unicode)
    * [Custom](#custom)
 * [Handling errors](#handling-errors)