Fix a small document issue (backport #13403)

[mergify]

Summary

This fixes a documentation issue as attached below.

(main branch)

https://docs.quantum.ibm.com/api/qiskit/transpiler_plugins#writing-plugins

(this PR)

Details and comments

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This is an automatic backport of pull request #13403 done by [Mergify](https://mergify.com).

[qiskit-bot]

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[coveralls]

Pull Request Test Coverage Report for Build 11719800781

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 31 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.04%) to 89.73%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/expr.rs	1	94.02%
qiskit/transpiler/passes/synthesis/unitary_synthesis.py	2	88.39%
crates/qasm2/src/lex.rs	4	92.23%
crates/qasm2/src/parse.rs	24	96.23%

Totals
Change from base Build 11618069198:	-0.04%
Covered Lines:	66873
Relevant Lines:	74527

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💛 - Coveralls