doc: add missing word

PR-URL: #14924
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Rich Trott <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: James M Snell <[email protected]>

Author: maclover7

BUILDING.md

@@ -134,7 +134,7 @@ 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