fix: add new line to prettify manifest fetch result

[qweeah]

So linux prompt will not be shown right after the prettified result.

[codecov-commenter]

Codecov Report

Merging #499 (f375b07) into main (7c6fbdb) will increase coverage by 16.21%.
The diff coverage is 80.92%.

@@             Coverage Diff             @@
##             main     #499       +/-   ##
===========================================
+ Coverage   55.69%   71.90%   +16.21%     
===========================================
  Files           6        9        +3     
  Lines         237      388      +151     
===========================================
+ Hits          132      279      +147     
+ Misses         90       87        -3     
- Partials       15       22        +7     

Impacted Files	Coverage Δ
internal/cache/target.go	72.46% <60.46%> (ø)
cmd/oras/internal/option/packer.go	81.25% <81.25%> (ø)
internal/cas/fetch.go	90.90% <90.90%> (ø)
cmd/oras/internal/option/platform.go	100.00% <100.00%> (ø)

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[shizhMSFT]

LGTM

[shizhMSFT]

LGTM