Update CI badge

Signed-off-by: Ry Jones <ry@linux.com>

README.md

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 [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://opensource.org/licenses/Apache-2.0)
 [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/960/badge)](https://bestpractices.coreinfrastructure.org/projects/960)
-[![Build Status](https://jenkins.soramitsu.co.jp/buildStatus/icon?job=iroha/iroha-hyperledger/master)](https://jenkins.soramitsu.co.jp/job/iroha/job/iroha-hyperledger/job/master/)
+[![Build Status](https://jenkins.soramitsu.co.jp/buildStatus/icon?job=iroha/iroha-hyperledger/main)](https://jenkins.soramitsu.co.jp/job/iroha/job/iroha-hyperledger/job/main/)
 
 Iroha is a straightforward distributed ledger technology (DLT), inspired by Japanese Kaizen principle — eliminate excessiveness (muri).
 Iroha has essential functionality for your asset, information and identity management needs, at the same time being an efficient and trustworthy crash fault-tolerant tool for your enterprise needs.