added one message at the beginning of dryRunPurge

[wangxiaoxuan273]

Purpose of the PR
#107 Improved the output message in dry run mode. Added one message making clear that nothing will be deleted, and changed the wording for dry run output.

Output after the change:

$ ./bin/acr purge \
--registry wxxfirstcontainerregistry.azurecr.io \
--filter "app:^hello.*" \
--ago 10d 

Number of deleted tags: 0
Number of deleted manifests: 0

$ ./bin/acr purge \
--registry wxxfirstcontainerregistry.azurecr.io \
--filter "app:^hello.*" \
--ago 10d \
--dry-run

DRY RUN: The following output shows what WOULD be deleted if the purge command was executed. Nothing is deleted.

Number of tags to be deleted: 0
Number of manifests to be deleted: 0

[shizhMSFT]

Instead of a claim at the very beginning, how about changing all the wordings for dry run?

For example,

• Tags to be deleted for repository: ...
• Number of tags to be deleted: ...
• Number of manifests to be deleted: ..

The above wording sounds less scary and can be distinguished from real purge easily.
Note: Lots of people read the logs from the bottom line to the top.

[shizhMSFT]

LGTM. It would be better to update the PR description with some tags / manifest to be deleted instead of 0s.

[northtyphoon]

LGTM