Add a prefix when replacing for doctest=:fix #2404

Job	Run time
NodeJS for prerender	1m 14s
Julia 1.6 - ubuntu-latest - x64	4m 41s
CSS for HTML themes	49s
Julia 1.6 - macos-latest - x64	8m 39s
PDF/LaTeX backend	3m 29s
Julia 1.6 - windows-latest - x64	16m 43s
Documentation: html	1m 13s
Linkcheck: online tests	2m 4s
Documentation: pdf	2m 48s
Linkcheck: Documenter manual	4m 17s
Julia 1 - ubuntu-latest - x64	3m 22s
Julia 1 - macos-latest - x64	7m 48s
Julia 1 - windows-latest - x64	5m 14s
Prettier (JS code formatting)	9s
Julia nightly - ubuntu-latest - x64	5m 32s
Julia nightly - macos-latest - x64	9m 49s
Julia nightly - windows-latest - x64	6m 12s
Julia 1 - ubuntu-latest - x86	3m 30s
	1h 27m 33s

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