Fix image name typo in ci job (Azure#18925)

Author: benbp

eng/pipelines/templates/jobs/archetype-sdk-client.yml

@@ -149,7 +149,7 @@ jobs:
       matrix:
         Linux:
           Pool: azsdk-pool-mms-ubuntu-1804-general
-          OSVmImage: MMSUbunut18.04
+          OSVmImage: MMSUbuntu18.04
           TestTargetFramework: netcoreapp2.1
         Windows_NetCoreApp:
           Pool: azsdk-pool-mms-win-2019-general # Comment out to swap back to public hosted pool.