Update PSMDB-DB Readme

[aminmokhtari94]

fix percona mongodb server image default tag value to 7.0.8-5

[it-percona-cla]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ tplavcic
❌ aminmokhtari94
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[jvpasinatto]

Hello @aminmokhtari94, please sign the CLA, bump the the chart version, and update the README. Thanks!

[jvpasinatto]

Closing as it was fixed in psmdb-db 1.17.0 release. Thank you @aminmokhtari94.