Missing backport for `generate_preset_pass_manager` docs (backport #12636)

[mergify[bot]]

This PR fixes the docs for the generate_preset_pass_manager method. The main branch was changed to say Preset pass managers instead of A preset pass manager, so I'm bringing this change to stable/1.1.

This will need backport to stable/0.46 as well.

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This is an automatic backport of pull request #12636 done by Mergify.

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[coveralls]

Pull Request Test Coverage Report for Build 9616575546

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 3 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.01%) to 87.197%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/expr.rs	1	94.02%
crates/qasm2/src/lex.rs	2	92.62%
<!--	Total:	3		-->

Totals
Change from base Build 9615556589:	0.01%
Covered Lines:	74507
Relevant Lines:	85447

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💛 - Coveralls