docs: make this line c&p-able (#96)

## Which problem is this PR solving?
I want to copy and paste this, so put it in a full code block

## Short description of the changes
- Update npm install command to use full code block syntax

## How to verify that this has the expected result
The command is more easily copy & paste able

README.md

@@ -42,7 +42,9 @@ This project provides a convenient distribution of all the code required to get
 
 Install this library:
 
-`npm install @honeycombio/opentelemetry-web @opentelemetry/auto-instrumentations-web`
+```sh
+npm install @honeycombio/opentelemetry-web @opentelemetry/auto-instrumentations-web
+```
 
 [Get a Honeycomb API key](https://docs.honeycomb.io/quickstart/#create-a-honeycomb-account).