Add curly brackets around $PWD usage to fix powershell issues

Signed-off-by: Michael Irwin <mikesir87@gmail.com>

Resolves #7

docs/tutorial/multi-container-apps/index.md

@@ -169,7 +169,7 @@ With all of that explained, let's start our dev-ready container!
 
     ```bash hl_lines="3 4 5 6 7"
     docker run -dp 3000:3000 \
-      -w /app -v $PWD:/app \
+      -w /app -v ${PWD}:/app \
       --network todo-app \
       -e MYSQL_HOST=mysql \
       -e MYSQL_USER=root \

docs/tutorial/using-bind-mounts/index.md

@@ -40,7 +40,7 @@ So, let's do it!
 
     ```bash
     docker run -dp 3000:3000 \
-        -w /app -v $PWD:/app \
+        -w /app -v ${PWD}:/app \
         node:12-alpine \
         sh -c "yarn install && yarn run dev"
     ```

docs/tutorial/using-docker-compose/index.md

@@ -53,7 +53,7 @@ To remember, this was the command we were using to define our app container.
 
 ```bash
 docker run -dp 3000:3000 \
-  -w /app -v $PWD:/app \
+  -w /app -v ${PWD}:/app \
   --network todo-app \
   -e MYSQL_HOST=mysql \
   -e MYSQL_USER=root \
@@ -102,7 +102,7 @@ docker run -dp 3000:3000 \
           - 3000:3000
     ```
 
-1. Next, we'll migrate both the working directory (`-w /app`) and the volume mapping (`-v $PWD:/app`) by using
+1. Next, we'll migrate both the working directory (`-w /app`) and the volume mapping (`-v ${PWD}:/app`) by using
    the `working_dir` and `volumes` definitions. Volumes also has a [short](https://docs.docker.com/compose/compose-file/#short-syntax-3) and [long](https://docs.docker.com/compose/compose-file/#long-syntax-3) syntax.
 
     One advantage of Docker Compose volume definitions is we can use relative paths from the current directory.