Fix Release version text

[t-kikuc]

What this PR does / why we need it:

~Since the site is launched by the following command,~ ~the RELEASE file should start with tag: vX.Y.Z.~ ~https://github.com/pipe-cd/pipecd/blob/daf0a09568c7333b8be11ad38988441eafe2b842/Makefile#L153~

Now: Generated is shown. (other docs too) ~

Which issue(s) this PR fixes:

N/A

Does this PR introduce a user-facing change?:

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[github-actions[bot]]

No GitHub releases will be created one this pull request got merged. Because the following tags were already created before.

• v0.48.8

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 22.86%. Comparing base (e7c40a0) to head (9612286). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #5192 +/- ## ======================================= Coverage 22.86% 22.86% ======================================= Files 420 420 Lines 45247 45247 ======================================= Hits 10344 10344 Misses 34108 34108 Partials 795 795 ```

:umbrella: View full report in Codecov by Sentry.
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[khanhtc1202]

@t-kikuc could you test with make release command?

[t-kikuc]

@khanhtc1202 I'm testing and fixing

[github-actions[bot]]

No GitHub releases will be created one this pull request got merged. Because the following tags were already created before.

• v0.48.8

[t-kikuc]

@khanhtc1202 I fixed 9e4cab5