chore(readme): update install code fences (#614)

README.md

@@ -35,17 +35,23 @@ Goose supports [embedding SQL migrations](#embedded-sql-migrations), which means
 
 # Install
 
-    $ go install github.com/pressly/goose/v3/cmd/goose@latest
+```shell
+go install github.com/pressly/goose/v3/cmd/goose@latest
+```
 
 This will install the `goose` binary to your `$GOPATH/bin` directory.
 
 For a lite version of the binary without DB connection dependent commands, use the exclusive build tags:
 
-    $ go build -tags='no_postgres no_mysql no_sqlite3' -o goose ./cmd/goose
+```shell
+go build -tags='no_postgres no_mysql no_sqlite3' -o goose ./cmd/goose
+```
 
 For macOS users `goose` is available as a [Homebrew Formulae](https://formulae.brew.sh/formula/goose#default):
 
-    $ brew install goose
+```shell
+brew install goose
+```
 
 See the docs for more [installation instructions](https://pressly.github.io/goose/installation/).