Docs: Fix a Typo in Intro to ART Document

In Enabling ART in Android Build section, correct a copy-paste typo 'libdvm.mk' -> 'libart.mk'

Change-Id: I30bcb43d137b75d3eb00caff5774f8a8ad62619b
Signed-off-by: Pipat Methavanitpong <pipat1010@gmail.com>
diff --git a/src/devices/tech/dalvik/art.jd b/src/devices/tech/dalvik/art.jd
index 902593e..33a82ed 100644
--- a/src/devices/tech/dalvik/art.jd
+++ b/src/devices/tech/dalvik/art.jd
@@ -210,7 +210,7 @@
 can be built using either or both runtimes. (You can dual boot from
 <strong>Developer options</strong> if both runtimes are installed.) See
 runtime_common.mk. That is included from build/target/product/runtime_libdvm.mk
-or build/target/product/runtime_libdvm.mk or both.</p>
+or build/target/product/runtime_libart.mk or both.</p>
 
 <p> The <code>dalvikvm</code> command line tool can run with either runtime now.
 It will default to using the runtime specified in <strong>developer