docs: fix .cer/.cert extension mismatch in Docker daemon trust config

[Adityakumar37]

What

Fixes goharbor/harbor#23534 — corrects a mismatch between the documented
certificate extension (.cert) and the extension actually produced by
the example OpenSSL command (.cer) in the "Configure the Docker Daemon
to Trust the Certificate" section of the HTTPS configuration guide.

Why

The docs state that the Docker daemon requires the .cert extension,
but the example command generates a .cer file:

openssl x509 -inform PEM -in yourdomain.com.crt -out yourdomain.com.cer

A reader who copies the command as written ends up with a .cer file,
which Docker's certs.d certificate loader does not recognize, while
the very next step in the same doc copies yourdomain.com.cert into
/etc/docker/certs.d/. This mismatch causes copy-paste failures that
surface later as an unrelated-looking
x509: certificate signed by unknown authority error.

Note for reviewers: older release branches (e.g. release-2.1.0,
release-2.3.0) have this command correct as .cert this appears to
be a regression introduced at some point on main.

Change

One-line fix: corrected the -out filename in the example command from
.cer to .cert, matching the surrounding prose and the subsequent
cp commands.

Additional context (re: issue's follow-up question)

The issue also asks whether .cert is a hard Docker requirement on
certificate content, or a naming convention. It's the latter
Docker's certs.d loader (/etc/docker/certs.d/<registry>/)
distinguishes files purely by filename suffix (ca.crt for CA certs,
*.cert for the client cert, *.key for the key); the underlying PEM
format is identical regardless of extension. Not adding this to the
doc body in this PR to keep the diff minimal per the incremental-PR
guidance in CONTRIBUTING.md — happy to add a clarifying note as a
follow-up if maintainers want it inline.

Closes goharbor/harbor#23534