README.md: update install notes

Update recommended installation documentation to use the "install"
target (from "install-headers").

Signed-off-by: James Knight <james.d.knight@live.com>
diff --git a/README.md b/README.md
index bca6722..3f76c0a 100644
--- a/README.md
+++ b/README.md
@@ -42,10 +42,7 @@
 mkdir build
 cd build
 cmake ..
-# Linux
-cmake --build . --target install-headers
-# Windows
-cmake --build . --config Debug --target install-headers
+cmake --build . --target install
 ```
 
 Then, for example, you will have `/usr/local/include/spirv/1.0/spirv.h`