Update path to fulcio-cert in verify example

Currently, the following error will be generated when running the
verify example:

Cannot build sigstore repo data: Error reading fulcio certificate from
disk: No such file or directory (os error 2)

This commit updates the path to point to fulcio.crt.pem in the same
directory as rekor.pub.

Signed-off-by: Daniel Bevenius <daniel.bevenius@gmail.com>

Author: danbev

examples/cosign/verify/README.md

@@ -27,6 +27,6 @@ Verify the image signature using the example program defined in
 cargo run --example verify -- \
   -k cosign.pub \
   --rekor-pub-key ~/.sigstore/root/targets/rekor.pub \
-  --fulcio-cert fulcio.crt.pem \
+  --fulcio-cert ~/.sigstore/root/targets/fulcio.crt.pem \
   registry-testing.svc.lan/busybox
 ```