Fix start span matchers slice flag

[jjti]

Fixing this flag being set to an empty string:

# josh @ M-FTQC4VW0R3 in ~/jjti/go-spancheck/testdata/base on git:main o [19:12:04] 
$ spancheck ./...    
2024/04/22 19:12:10 [WARN] invalid start span signature "". expected regex:telemetry-type
/Users/josh/jjti/go-spancheck/testdata/base/base.go:22:2: span is unassigned, probable memory leak
/Users/josh/jjti/go-spancheck/testdata/base/base.go:23:7: span is unassigned, probable memory leak
/Users/josh/jjti/go-spancheck/testdata/base/base.go:28:2: span.End is not called on all paths, possible memory leak
/Users/josh/jjti/go-spancheck/testdata/base/base.go:30:1: return can be reached without calling span.End

[coveralls]

Pull Request Test Coverage Report for Build 8792192493

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 83.942%

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Totals
Change from base Build 8774900976:	0.0%
Covered Lines:	345
Relevant Lines:	411

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💛 - Coveralls