Remove unneeded + sign (#42149)

Co-authored-by: William Godbe <wigodbe@microsoft.com>

.azure/pipelines/azure-pipelines-mirror-within-azdo.yml

@@ -38,7 +38,7 @@ jobs:
             $branch = "$(Build.SourceBranch)".Replace("refs/heads/", "");
             $suffix = "-nonstable"
             Write-Host "##vso[task.setvariable variable=BranchToMirror]$branch"
-            Write-Host "##vso[task.setvariable variable=TargetBranchName]$branch+$suffix"
+            Write-Host "##vso[task.setvariable variable=TargetBranchName]$branch$suffix"
           displayName: Calculate Mirrored Branch Names
         - script: |
             git clone https://dn-bot:$(dn-bot-dnceng-build-rw-code-rw)@dev.azure.com/dnceng/internal/_git/$(AzdoRepo) $(WorkingDirectoryName) --recursive --no-tags --branch $(TargetBranchName)