Update osx.mk - Added "apple" to USE_BLAS comment (apache#12819)

Added "apple" to USE_BLAS comment because it is one of the versions that are possible. 
Currently the comment only has "mkl, blas, atlas, openblas" that can be used

Author: rahul3

make/osx.mk

@@ -80,7 +80,7 @@ USE_OPENCV = 1
 USE_OPENMP = 0
 
 # choose the version of blas you want to use
-# can be: mkl, blas, atlas, openblas
+# can be: mkl, blas, atlas, openblas, apple
 USE_BLAS = apple
 
 # whether use lapack during compilation