Add npm as a dependency for macOS

npm is a dependency, and it is listed as such for Ubuntu, but not
for macOS.

I could not get the pebble SDK installed until I had npm. Since we
are being directed to use homebrew, and it can install npm, do that.

Author: a1291762

source/sdk/index.md

@@ -30,7 +30,7 @@ scripts:
 You'll need Python 3.10 or above, not the MacOS default installation of 3.9. To install a newer version of Python using [Homebrew](https://brew.sh/), run:
 
 ```bash
-brew install python
+brew install python npm
 ```
 
 #### Ubuntu