Merge #12586: docs: Update osx brew install instruction

59f47959b docs: Update osx brew install instruction (fanquake)

Pull request description:

  Python 3.x is now the default python formula in [homebrew](https://github.com/Homebrew/homebrew-core/blob/master/Formula/python.rb).
  https://brew.sh/2018/01/19/homebrew-1.5.0/ has some more info.

Tree-SHA512: f684019126d38debe897287b4bd9803b1ced2c32f66230a8a5eb468759cbec170b9367648bd7ba6dc4ea9489aa85a1b2f0445c384bbc5bf76d18073564f80b59

doc/build-osx.md

@@ -16,7 +16,7 @@ Then install [Homebrew](https://brew.sh).
 Dependencies
 ----------------------
 
-    brew install automake berkeley-db4 libtool boost miniupnpc openssl pkg-config protobuf python3 qt libevent
+    brew install automake berkeley-db4 libtool boost miniupnpc openssl pkg-config protobuf python qt libevent
 
 See [dependencies.md](dependencies.md) for a complete overview.