arm: fix mention of armrc in caveats

Placing your configuration in `~/.arm/.armrc` does not work.

The correct place to keep the configuration file is `~/.arm/armrc`, as
can be seen here:
https://github.com/flamsmark/arm/blob/e2e3091c1af2741e046831fdecd73677b3c1652b/src/starter.py#L35

Closes #32126.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>