docs: Fix the documentation.

docs/index.md

 
 - [Features](#features)
 - [History](#history)
-- [Adding a Language](add_language.md)
+- [Adding or Improving a Language](add_language.md)
   - [Text to Phoneme Translation](dictionary.md)
 - [Voice Files](voices.md)
   - [MBROLA Voices](mbrola.md)
   - [Phoneme Tables](phontab.md)
 - [Intonation](intonation.md)
-- [Markup Tags](markup.md)
+- [Text Markup](markup.md)
 - [License](../COPYING)
 
 ----------
 build system to autotools in 2012. In late 2015, this project was officially
 forked to a new __eSpeak NG__ project. The new eSpeak NG project is a significant
 departure from the eSpeak project, with the intention of cleaning up the
-existing codebase, adding new features and adding and improving to the
+existing codebase, adding new features, and adding to and improving the
 supported languages.

docs/voices.md

   - [intonation](#intonation)
   - [dictmin](#dictmin)
   - [letterVowel](#letterVowel)
+
 ----------
 
 A Voice file specifies a language (and possibly a language variant or
 grouped by the [ISO 639-5](https://en.wikipedia.org/wiki/ISO_639-5)
 language family of the language being specified in the voice files.
 See also Wikipedia's
-[List of language families] (https://en.wiktionary.org/wiki/Wiktionary:List_of_families)
+[List of language families](https://en.wiktionary.org/wiki/Wiktionary:List_of_families)
 for more details.
 
 The `default` voice is used if none is specified in the speak command. You