doc: add missing word

PR-URL: https://github.com/nodejs/node/pull/14924
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

BUILDING.md

@@ -132,7 +132,7 @@ This will build Node.js first (if necessary) and then use it to build the docs:
 $ make doc
 ```
 
-If you have an existing Node.js you can build just the docs with:
+If you have an existing Node.js build, you can build just the docs with:
 
 ```console
 $ NODE=/path/to/node make doc-only