Add a prefix when replacing for doctest=:fix #2373

Job	Run time
PDF/LaTeX backend	3m 41s
Documentation: html	1m 4s
Documentation: pdf	2m 57s
Linkcheck: online tests	1m 52s
NodeJS for prerender	1m 1s
CSS for HTML themes	54s
Julia 1.6 - ubuntu-latest - x64	4m 13s
Julia 1.6 - macos-latest - x64	7m 22s
Linkcheck: Documenter manual	4m 17s
Julia 1.6 - windows-latest - x64	16m 6s
Julia 1 - ubuntu-latest - x64	3m 13s
Prettier (JS code formatting)	4s
Julia 1 - macos-latest - x64	5m 28s
Julia 1 - windows-latest - x64	4m 41s
Julia nightly - ubuntu-latest - x64	4m 33s
Julia nightly - macos-latest - x64	8m 30s
Julia nightly - windows-latest - x64	6m 2s
Julia 1 - ubuntu-latest - x86	4m 14s
	1h 20m 12s

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