doc: fix typo in buffer module documentation

PR-URL: #1169
Reviewed-By: Ben Noordhuis <[email protected]>

Author: yursha

doc/api/buffer.markdown

@@ -874,5 +874,5 @@ un-pooled Buffer instance using SlowBuffer and copy out the relevant bits.
       store.push(sb);
     });
 
-Though this should used sparingly and only be a last resort *after* a developer
+Though this should be used sparingly and only be a last resort *after* a developer
 has actively observed undue memory retention in their applications.