adding cd command to mkcert instructions (#690)

to clarify that the cert needs to be placed in the web folder so that webpack will use it

Author: kdenney

docs/getting-started/clients/web-vault/index.mdx

@@ -22,6 +22,7 @@ support WebAuthn which is blocked on websites with certificate errors. Please fo
 mkcert --install
 
 # Generate cert valid for localhost, 127.0.0.1 and bitwarden.test
+cd apps/web
 mkcert -cert-file dev-server.local.pem -key-file dev-server.local.pem localhost 127.0.0.1 bitwarden.test
 ```