fix(docs): clean up npm prefix docs

Adds an example and slightly changes wording

PR-URL: #2463
Credit: @wraithgar
Close: #2463
Reviewed-by: @darcyclarke

docs/content/commands/npm-prefix.md

@@ -12,13 +12,25 @@ npm prefix [-g]
 
 ### Description
 
-Print the local prefix to standard out. This is the closest parent directory
+Print the local prefix to standard output. This is the closest parent directory
 to contain a `package.json` file or `node_modules` directory, unless `-g` is
 also specified.
 
 If `-g` is specified, this will be the value of the global prefix. See
 [`npm config`](/commands/npm-config) for more detail.
 
+### Example
+
+```bash
+npm prefix
+/usr/local/projects/foo
+```
+
+```bash
+npm prefix -g
+/usr/local
+```
+
 ### See Also
 
 * [npm root](/commands/npm-root)