Fix GitHub source code links on Windows docs builds (backport #13292)

[mergify]

Per https://docs.python.org/3/library/pathlib.html#pathlib.PurePath.as_posix, calling str() on a Windows Path object uses \ as the separator. We need to use / for the GitHub URL to work properly.

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This is an automatic backport of pull request #13292 done by Mergify.

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[coveralls]

Pull Request Test Coverage Report for Build 11224556151

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 6 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.003%) to 89.747%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/expr.rs	1	94.02%
crates/qasm2/src/lex.rs	5	92.48%

Totals
Change from base Build 11219525131:	-0.003%
Covered Lines:	66885
Relevant Lines:	74526

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💛 - Coveralls