Fix documentation for importing header only target with CMake

[sdebionne]

I agree that my contributions are licensed under the {fmt} license, and agree to future changes to the licensing.

[vitaut]

Thanks for the PR, but fmt-header-only is an alias to fmt::fmt-header-only so the change is unnecessary:

fmt/CMakeLists.txt

Line 200 in 9f09b8e

add_library(fmt::fmt-header-only ALIAS fmt-header-only)

[sdebionne]

When using fmt with find_package(), that is as an imported target, I think that fmt::fmt-header-only has to be used since :: tells CMake that it is an imported target. I came up with this PR after trying the following from the doc:

find_package(fmt)
target_link_libraries(<your-target> PRIVATE fmt-header-only)

This fails on my setup with a link error on the line of "unable to find fmt-header-only.so". But no big deal...

[vitaut]

This fails on my setup with a link error on the line of "unable to find fmt-header-only.so".

Interesting. Let's merge this then.