Switch docs to recommending NASM.

Chromium has now switched to building our assembly with NASM
(https://crbug.com/766721), which is more maintained. Next step
is to switch remaining folks (Conscrypt, not sure if there's anyone
else) and we'll drop Yasm.

Change-Id: If4f45399b48d0d7477afb47647e83e7250bf854f
Reviewed-on: https://boringssl-review.googlesource.com/c/33144
Reviewed-by: Adam Langley <agl@google.com>

BUILDING.md

@@ -20,7 +20,7 @@
   * If you need to build Ninja from source, then a recent version of
     [Python](https://www.python.org/downloads/) is required (Python 2.7.5 works).
 
-  * On Windows only, [Yasm](http://yasm.tortall.net/) is required. If not found
+  * On Windows only, [NASM](https://www.nasm.us/) is required. If not found
     by CMake, it may be configured explicitly by setting
     `CMAKE_ASM_NASM_COMPILER`.