fix brew install command to include --cask flag (#56)

The tap distributes kage as a Cask (prebuilt binary), not a Formula,
so brew needs --cask to find it. Without the flag brew errors out with
"No available formula or cask with the name kage".

README.md

@@ -28,7 +28,7 @@ Prefer a prebuilt binary? Grab an archive, a `.deb`/`.rpm`/`.apk`, or a checksum
 
 ```bash
 # Homebrew (macOS)
-brew install tamnd/tap/kage
+brew install --cask tamnd/tap/kage
 
 # Scoop (Windows)
 scoop bucket add tamnd https://github.com/tamnd/scoop-bucket

docs/content/getting-started/installation.md

@@ -15,7 +15,7 @@ go install github.com/tamnd/kage/cmd/kage@latest
 ## Homebrew (macOS)
 
 ```bash
-brew install tamnd/tap/kage
+brew install --cask tamnd/tap/kage
 ```
 
 The cask installs the prebuilt macOS binary. On Linux, use the packages below or