Fixed pnpm reference

Author: mattpocock

Diff for: README.md

@@ -37,8 +37,8 @@ Say thanks to Matt on [Twitter](https://twitter.com/mattpocockuk) or by joining
 
 ## Reference
 
-### `pnpm run exercise`
+### `npm run exercise`
 
-Alias: `pnpm run e`
+Alias: `npm run e`
 
 Open a prompt for choosing which exercise you'd like to run.