docs: Remove osx-specific func from Linux instrs

Fixed a pre-existing typo in the android instructions for Linux.

Change-Id: I7ceea88d07a69a60fc1afe4b2facf3e03f0da0e5
diff --git a/BUILD.md b/BUILD.md
index c216852..731f717 100644
--- a/BUILD.md
+++ b/BUILD.md
@@ -389,7 +389,7 @@
 cd build-android
 ./update_external_sources_android.sh --no-build
 ./android-generate.sh
-ndk-build -j $(sysctl -n hw.ncpu)
+ndk-build -j4
 ```
 #### Windows
 Follow the setup steps for Windows above, then from Developer Command Prompt for VS2013: