Adding symbol to the WorkflowMultiBranchProject class descriptor

[joseblas]

The WorkflowMultiBranchProject class does not have a proper Symbol in the descriptor as many other such as FreeStyleProject (https://github.com/jenkinsci/jenkins/blob/master/core/src/main/java/hudson/model/FreeStyleProject.java#L79)

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[jglick]

Note that the display name currently specifies Pipeline

workflow-multibranch-plugin/src/main/resources/org/jenkinsci/plugins/workflow/multibranch/Messages.properties

Line 2 in e4aa6a8

WorkflowMultiBranchProject.DisplayName=Multibranch Pipeline

OTOH there is no other seriously maintained multibranch impl (literate plugin is long dead, as is yaml-project) so probably just multibranch is clear enough.