doc: "a" -> "an" in api/documentation.md

Fixes: https://github.com/nodejs/node/issues/6642
PR-URL: https://github.com/nodejs/node/pull/6689
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Alexander Makarenko <estliberitas@gmail.com>
Reviewed-By: Jeremiah Senkpiel <Fishrock123@rocketmail.com>

doc/api/documentation.md

@@ -21,7 +21,7 @@ documentation is generated using the `tools/doc/generate.js` program.
 The HTML template is located at `doc/template.html`.
 
 
-If you find a error in this documentation, please [submit an issue][]
+If you find an error in this documentation, please [submit an issue][]
 or see [the contributing guide][] for directions on how to submit a patch.
 
 ## Stability Index