misc/android: fix a typo in README

A cross C compiler should be specified when cross building go
for android on x86 host. This CL fixes the typo in README.

Change-Id: Ideda915364d9f7f0fcd53146abc1e74506a8a8e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/177777Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Elias Naur <mail@eliasnaur.com>

misc/android/README

@@ -12,7 +12,7 @@ C compiler from the Android NDK. For example,
 	CGO_ENABLED=1 \
 	GOOS=android \
 	GOARCH=arm64 \
-	CC=$NDK/toolchains/llvm/prebuilt/linux-x86_64/bin/aarch64-linux-android21-clang \
+	CC_FOR_TARGET=$NDK/toolchains/llvm/prebuilt/linux-x86_64/bin/aarch64-linux-android21-clang \
 	./all.bash
 
 To run tests on the Android device, add the bin directory to PATH so the