Add spaces to deprecation warning about qiskit.providers.models (backport #12894)

[mergify]

Summary

This adds some spaces that are currently missing in a deprecation warning.

Details and comments

I reviewed the other warnings added as part of the same PR (#12629), and all the others looked correct to me.

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This is an automatic backport of pull request #12894 done by Mergify.

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[coveralls]

Pull Request Test Coverage Report for Build 10220526476

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 9 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.001%) to 89.934%

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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	91.73%
crates/qasm2/src/parse.rs	6	96.69%

Totals
Change from base Build 10218811685:	0.001%
Covered Lines:	66733
Relevant Lines:	74202

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💛 - Coveralls