[Ci]: Fix an azp template syntax error (#10446)

Why I did it
[Ci]: Fix an azp template syntax error
See https://dev.azure.com/mssonic/build/_build/results?buildId=86535&view=results

/.azure-pipelines/docker-sonic-slave.yml (Line: 62, Col: 47): Expected a mapping

.azure-pipelines/docker-sonic-slave.yml

@@ -60,15 +60,15 @@ stages:
   - ${{ each dist in parameters.dists }}:
     - ${{ if endswith(variables['Build.DefinitionName'], dist) }}:
       - ${{ each arch in parameters.arches }}:
-        ${{ if eq(variables['System.PullRequest.TargetBranch'], 'master') }}:
-        - template: docker-sonic-slave-template.yml
-          parameters:
-            pool: sonicbld
-            arch: ${{ arch }}
-            dist: ${{ dist }}
-        ${{ else }}:
-        - template: .azure-pipelines/docker-sonic-slave-template.yml@buildimage
-          parameters:
-            pool: sonicbld
-            arch: ${{ arch }}
-            dist: ${{ dist }}
+        - ${{ if eq(variables['System.PullRequest.TargetBranch'], 'master') }}:
+          - template: docker-sonic-slave-template.yml
+            parameters:
+              pool: sonicbld
+              arch: ${{ arch }}
+              dist: ${{ dist }}
+        - ${{ else }}:
+          - template: .azure-pipelines/docker-sonic-slave-template.yml@buildimage
+            parameters:
+              pool: sonicbld
+              arch: ${{ arch }}
+              dist: ${{ dist }}