Fix fish shell config docs

[BooleanCat]

Make sure you have checked all steps below.

Prerequisite

• Please consider implementing the feature as a hook script or plugin as a first step.
• Please consider contributing the patch upstream to rbenv, since we have borrowed most of the code from that project.
• My PR addresses the following pyenv issue (if any)

Description

• Here are some details about my PR

Copying the instructions for configuring the fish shell for pyenv verbatim fails because fish_add_path fails when the target directory doesn't exist.

Tests

• My PR adds the following unit tests (if any)

N/A - doc change only.

[native-api]

It's pointless to create $PYENV_ROOT/bin if it doesn't exist.
The command should then be conditional on the directory existing.

Same goes for the 2nd snippet just below.