change README according to pr comments

Author: mxie32

README.md

@@ -6,12 +6,12 @@ INSTALL
 =======
 - Python3 is required
 - Make the following changes before build and install:
-    - enable `GTSAM_INSTALL_CYTHON_TOOLBOX`
-    - if you want to build the gtsam python library for a specific python version (eg 3.6), use the `-DGTSAM_PYTHON_VERSION=3.6` option, otherwise the default interpreter will be used.
+    - Set the CMake flag `GTSAM_INSTALL_CYTHON_TOOLBOX=ON` to enable building the cython wrapper.
+    - Set the CMake flag `-DGTSAM_PYTHON_VERSION=3`, otherwise the default interpreter will be used.
 - Run `cmake ..`
 - Run `make install` (may need use sudo), and the wrapped module will be installed to `GTSAM_CYTHON_INSTALL_PATH`, which is
 by default: `<your CMAKE_INSTALL_PREFIX>/cython`
-- After build and install gtsam, navigate to `GTSAM_CYTHON_INSTALL_PATH` and run `python setup.py install`
+- After building and installing gtsam, navigate to `GTSAM_CYTHON_INSTALL_PATH` and run `python setup.py install`
 
 For more detailed information, please see 
 https://github.com/borglab/gtsam/tree/develop/cython