Fix documentation bug

Author: ryanleary

src/configure

@@ -22,7 +22,7 @@
 # ./configure --atlas-root=../tools/ATLAS/build
 # ./configure --use-cuda=no   # disable CUDA detection (will build cpu-only
 #                             # version of kaldi even on CUDA-enabled machine
-# ./configure --use-cuda --cudatk-dir=/usr/local/cuda/ --cuda-arch=-sm_70
+# ./configure --use-cuda --cudatk-dir=/usr/local/cuda/ --cuda-arch=-arch=sm_70
 #        # Use cuda in /usr/local/cuda and set the arch to -sm_70
 # ./configure --static --fst-root=/opt/cross/armv8hf \
 # --atlas-root=/opt/cross/armv8hf --host=armv8-rpi3-linux-gnueabihf