[readme] Fix /opt mistake.

I previously edited the mac section when I had meant to edit the ubuntu section.

Author: chrisdembia

README.md

@@ -273,7 +273,7 @@ On Mac using Xcode
    changing the `CMAKE_INSTALL_PREFIX` variable. We'll assume you set it to
    `~/opensim-core`. If you choose a different installation location, make
    sure to use *yours* where we use `~/opensim-core` below. You should *not*
-   use `/usr/`, `/usr/local/` (because our installation does not yet conform to the [FHS](http://www.pathname.com/fhs/)), but [`/opt/`](http://www.tldp.org/LDP/Linux-Filesystem-Hierarchy/html/opt.html) is okay.
+   use `/usr/`, `/usr/local/` (because our installation does not yet conform to the [FHS](http://www.pathname.com/fhs/)).
 6. Tell CMake where you installed Simbody by setting the `SIMBODY_HOME`
    variable to where you installed Simbody (e.g., `~/simbody`). If you
    installed Simbody using `brew`, then CMake will find Simbody automatically.
@@ -401,7 +401,7 @@ And you could get all the optional dependencies via:
    changing the `CMAKE_INSTALL_PREFIX` variable. We'll assume you set it to
    `~/opensim-core`. If you choose a different installation location, make
    sure to use *yours* where we use `~/opensim-core` below. You should *not*
-   use `/usr/`, `/usr/local/`, or any typical system path.
+   use `/usr/`, `/usr/local/` (because our installation does not yet conform to the [FHS](http://www.pathname.com/fhs/)), but [`/opt/`](http://www.tldp.org/LDP/Linux-Filesystem-Hierarchy/html/opt.html) is okay.
 6. Tell CMake where you installed Simbody by setting the `SIMBODY_HOME`
    variable to where you installed Simbody (e.g., `~/simbody`). If you
    installed Simbody using `brew`, then CMake will find Simbody automatically.