chsh docs: look up fish in $PATH instead of /usr/local/bin

Now that the « chsh -s $(command -v) » approach should work both
in and outside fish, it seems like we should use that.

Non-macOS users probably shouldn't do this, but there's already a
big warning above this section.

Fixes #11931

doc_src/index.rst

@@ -63,7 +63,7 @@ Default Shell
 
 There are multiple ways to switch to fish (or any other shell) as your default.
 
-The simplest method is to set your terminal emulator (eg GNOME Terminal, Apple's Terminal.app, or Konsole) to start fish directly. See its configuration and set the program to start to ``/usr/local/bin/fish`` (if that's where fish is installed - substitute another location as appropriate).
+The simplest method is to set your terminal emulator (e.g. GNOME Terminal, Apple's Terminal.app, or Konsole) to start fish directly. See its configuration and set the program to start to ``/usr/local/bin/fish`` (the exact path depends on how you installed fish).
 
 Alternatively, you can set fish as your login shell so that it will be started by all terminal logins, including SSH.
 
@@ -75,13 +75,13 @@ To change your login shell to fish:
 
 1. Add the shell to ``/etc/shells`` with::
 
-    > echo /usr/local/bin/fish | sudo tee -a /etc/shells
+    > command -v fish | sudo tee -a /etc/shells
 
 2. Change your default shell with::
 
-    > chsh -s /usr/local/bin/fish
+    > chsh -s "$(command -v fish)"
 
-Again, substitute the path to fish for ``/usr/local/bin/fish`` - see ``command -s fish`` inside fish. To change it back to another shell, just substitute ``/usr/local/bin/fish`` with ``/bin/bash``, ``/bin/tcsh`` or ``/bin/zsh`` as appropriate in the steps above.
+To change it back to another shell, substitute ``fish`` with ``bash``, ``tcsh`` or ``zsh`` as appropriate in the above command.
 
 Uninstalling
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