don't ask user to install python2 (#1136)

Summary: Pull Request resolved: #1136

Test Plan:
<!--
  Demonstrate the code is solid.
  Example: The exact commands you ran and their output,
  screenshots / videos if the pull request changes the user interface.
-->

Reviewed By: tmikov

Differential Revision: D49843779

Pulled By: avp

fbshipit-source-id: 2b9fb1a77c805fb4fc078f42e85a806e5646f79e

doc/BuildingAndRunning.md

@@ -13,7 +13,7 @@ The Hermes REPL will also use libreadline, if available.
 
 To install dependencies on Ubuntu:
 
-    apt install cmake git ninja-build libicu-dev python zip libreadline-dev
+    apt install cmake git ninja-build libicu-dev python3 zip libreadline-dev
 
 On Arch Linux: