Remove duplicate help tags in the docs

From `:help help-writing`

> To define a help tag, place the name between asterisks (*tag-name*).
> The tag-name should be different from all the Vim help tag names and
> ideally should begin with the name of the Vim plugin.  The tag name is
> usually right aligned on a line.

Fixes vim startup errors with pathogen.
Merge pull request #1 from acrisci/fix-duplicate-tags

Remove duplicate help tags in the docs