Update appmesh.md

[ricardo-aspira]

Replace the following command:

aws appmesh delete-route \
          --mesh appmesh-workshop \
          --virtual-router-name $virtualRouterName \
          --route-name $routeName

By:

aws appmesh delete-route \
          --mesh-name appmesh-workshop \
          --virtual-router-name $virtualRouterName \
          --route-name $routeName

Issue #, if available: Not able to delete the routes due to parameter error.

Description of changes: Changed --mesh by --mesh-name.

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