Docs: Bump OSX version to 10.13 in build-osx.md

Author: Varunram

doc/build-osx.md

@@ -110,6 +110,6 @@ Uncheck everything except Qt Creator during the installation process.
 Notes
 -----
 
-* Tested on OS X 10.8 through 10.12 on 64-bit Intel processors only.
+* Tested on OS X 10.8 through 10.13 on 64-bit Intel processors only.
 
 * Building with downloaded Qt binaries is not officially supported. See the notes in [#7714](https://github.com/bitcoin/bitcoin/issues/7714)