README: add "cd mkcert" to build from source instructions (#228)

Co-authored-by: Filippo Valsorda <1225294+FiloSottile@users.noreply.github.com>

README.md

@@ -72,7 +72,7 @@ brew install mkcert
 or build from source (requires Go 1.13+)
 
 ```
-git clone https://github.com/FiloSottile/mkcert
+git clone https://github.com/FiloSottile/mkcert && cd mkcert
 go build -ldflags "-X main.Version=$(git describe --tags)"
 ```