fix: swaggerinfo template for instanceNames

[Syuparn]

Describe the PR
e.g. fix template to generate swaggerInfo variable when --instanceName option is specified

Relation issue

#1133

[ubogdan]

LGTM

[ubogdan]

@Syuparn Would you mind adding a unit test for template rendering output to cover this fix?

[codecov]

Codecov Report

Merging #1134 (87a827a) into master (7e207e5) will not change coverage.
The diff coverage is n/a.

❗ Current head 87a827a differs from pull request most recent head 9b56776. Consider uploading reports for the commit 9b56776 to get more accurate results

@@           Coverage Diff           @@
##           master    #1134   +/-   ##
=======================================
  Coverage   94.73%   94.73%           
=======================================
  Files          10       10           
  Lines        2433     2433           
=======================================
  Hits         2305     2305           
  Misses         67       67           
  Partials       61       61           

Impacted Files	Coverage Δ
gen/gen.go	93.33% <ø> (ø)

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[Syuparn]

@Syuparn Would you mind adding a unit test for template rendering output to cover this fix?

@ubogdan

I added variable rendering tests.

[ubogdan]

LGTM

[ubogdan]

@Syuparn Thanks for your contribution