README: fix EL8 go-md2man install

Currently EL8 comes with Go 1.22.x, which means "go install"
(rather than "go get") should be used.

Also,
 - there is no need to set GOPATH;
 - `go env GOPATH` can be used to determine the built-in default.

Tested on AlmaLinux 8.10.

Signed-off-by: Kir Kolyshkin <[email protected]>

README.md

@@ -76,9 +76,8 @@ installed with:
 
 ```console
 $ sudo yum --enablerepo='*' install -y golang
-$ export GOPATH=$HOME/go
-$ go get github.com/cpuguy83/go-md2man
-$ export PATH=$PATH:$GOPATH/bin
+$ go install github.com/cpuguy83/go-md2man@latest
+$ export PATH=$PATH:$(go env GOPATH)/bin
 ```
 
 ### Ubuntu