docs: update Fish PATH update

[gregorias]

Prerequisite

• Please consider implementing the feature as a hook script or plugin as a first step.
  • pyenv has some powerful support for plugins and hook scripts. Please refer to Authoring plugins for details and try to implement it as a plugin if possible.
• Please consider contributing the patch upstream to rbenv, since we have borrowed most of the code from that project.
  • We occasionally import the changes from rbenv. In general, you can expect changes made in rbenv will be imported to pyenv too, eventually.
  • Generally speaking, we prefer not to make changes in the core in order to keep compatibility with rbenv.
• My PR addresses the following pyenv issue (if any)
  • Closes https://github.com/pyenv/pyenv/issues/XXXX

Description

The original instructions were added 2 years ago in 0f2d659. Since then, Fish released a more declarative instruction for this operation, which is now the recommended way of manipulating PATH in Fish.