add syntax highlighting for code blocks

Signed-off-by: Feynman Zhou <[email protected]>

docs/CLI/index.md

@@ -6,15 +6,15 @@ This guide demonstrates the installation steps of ORAS CLI on different platform
 
 Install `oras` using [Homebrew](https://brew.sh/):
 
-```
+```bash
 brew install oras
 ```
 
 ## GoFish
 
 Install `oras` using [GoFish](https://gofi.sh/):
 
-```
+```bash
 gofish install oras
 ```
 
@@ -24,17 +24,17 @@ Install from the latest [release artifacts](https://github.com/oras-project/oras
 
 ### Linux
 
-```
+```bash
 curl -LO https://github.com/oras-project/oras/releases/download/v0.12.0/oras_0.12.0_linux_amd64.tar.gz
 mkdir -p oras-install/
 tar -zxf oras_0.12.0_*.tar.gz -C oras-install/
 mv oras-install/oras /usr/local/bin/
 rm -rf oras_0.12.0_*.tar.gz oras-install/
 ```
 
-### MacOS
+### macOS
 
-```
+```bash
 curl -LO https://github.com/oras-project/oras/releases/download/v0.12.0/oras_0.12.0_darwin_amd64.tar.gz
 mkdir -p oras-install/
 tar -zxf oras_0.12.0_*.tar.gz -C oras-install/
@@ -46,7 +46,7 @@ rm -rf oras_0.12.0_*.tar.gz oras-install/
 
 Add `%USERPROFILE%\bin\` to your `PATH` environment variable so that `oras.exe` can be found.
 
-```
+```shell
 curl.exe -sLO  https://github.com/oras-project/oras/releases/download/v0.12.0/oras_0.12.0_windows_amd64.tar.gz
 tar.exe -xvzf oras_0.12.0_windows_amd64.tar.gz
 mkdir -p %USERPROFILE%\bin\
@@ -64,10 +64,10 @@ docker run -it --rm -v $(pwd):/workspace ghcr.io/oras-project/oras:v0.12.0 help
 
 > Note: the default WORKDIR  in the image is `/workspace`.
 
-## Verify 
+## Verify
 
-```
-oras version
+```shell
+$ oras version
 Version:        0.12.0
 Go version:     go1.16.3
 Git commit:     1e6a64e1789f5145bf669b75bebfe013100f6253