[docs] freebsd does not have pkg-config since FBSD12 (#222)

Signed-off-by: lizzie <lizzie@eden-emu.dev>
Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/222
Reviewed-by: crueter <crueter@eden-emu.dev>
Co-authored-by: lizzie <lizzie@eden-emu.dev>
Co-committed-by: lizzie <lizzie@eden-emu.dev>

docs/build/FreeBSD.md

@@ -18,7 +18,7 @@ devel/nlohmann-json
 devel/ninja
 devel/nasm
 devel/autoconf
-devel/pkg-config  
+devel/pkgconf
 devel/qt6-base
 
 multimedia/ffnvcodec-headers
@@ -34,6 +34,8 @@ graphics/glslang
 graphics/vulkan-utility-libraries
 ```
 
+If using FreeBSD 12 or prior, use `devel/pkg-config` instead.
+
 ---
 
 ### Build preparations: