[skip ci] minor update

install.md

@@ -4,7 +4,7 @@
 
 - Ubuntu 18.04 or higher
 
-  If your Ubuntu version is lower than 18.04, we recommend you to pull a [docker image of Ubuntu 18.04](https://docs.docker.com/install/linux/docker-ce/ubuntu/) as your compilation environment.
+  If your operating system is not Ubuntu 18.04 or higher, we recommend you to pull a [docker image of Ubuntu 18.04](https://docs.docker.com/install/linux/docker-ce/ubuntu/) as your compilation environment.
   
 - CMake 3.12 or higher