doc: Small change to documentation in DectectPython.cmake

Fixed small typo for python 3 default version.
Signed-off-by: Noel Eck <noel.eck@intel.com>

cmake/OpenCVDetectPython.cmake

@@ -244,7 +244,7 @@ if(PYTHON_DEFAULT_EXECUTABLE)
 elseif(PYTHON2INTERP_FOUND) # Use Python 2 as default Python interpreter
     set(PYTHON_DEFAULT_AVAILABLE "TRUE")
     set(PYTHON_DEFAULT_EXECUTABLE "${PYTHON2_EXECUTABLE}")
-elseif(PYTHON3INTERP_FOUND) # Use Python 2 as fallback Python interpreter (if there is no Python 2)
+elseif(PYTHON3INTERP_FOUND) # Use Python 3 as fallback Python interpreter (if there is no Python 2)
     set(PYTHON_DEFAULT_AVAILABLE "TRUE")
     set(PYTHON_DEFAULT_EXECUTABLE "${PYTHON3_EXECUTABLE}")
 endif()