doc: fix documentation reference

Intel Processor Trace is documented in ยง35 of Vol-3C of the Intel SDM.  Fix
the outdated link in the getting started documentation.  We remain a bit
vague with respect to the actual chapter number in case it moves.

Fixes #54.

Change-Id: I5f06e9f590f269f29dc46d10d12392f4f7992218
Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>
diff --git a/doc/getting_started.md b/doc/getting_started.md
index cc847ea..b8cbbcc 100644
--- a/doc/getting_started.md
+++ b/doc/getting_started.md
@@ -29,12 +29,14 @@
  ! POSSIBILITY OF SUCH DAMAGE.
  !-->
 
-This chapter gives a brief introduction into the sample tools using one of the
-tests as example.  It assumes that you are already familiar with Intel(R)
-Processor Trace (Intel PT) and that you already built the decoder library and
-the sample tools.  For detailed information about Intel PT, please refer to
-chapter 11 of the Intel Architecture Instruction Set Extensions Programming
-Reference at http://www.intel.com/products/processor/manuals/.
+This chapter gives a brief introduction into the sample tools using one of
+the tests as example.  It assumes that you are already familiar with
+Intel(R) Processor Trace (Intel PT) and that you already built the decoder
+library and the sample tools.
+
+For detailed information about Intel PT, please refer to the respective
+chapter in Volume 3 of the Intel Software Developer's Manual at
+https://www.intel.com/sdm.
 
 Start by compiling the loop-tnt test.  It consists of a small assembly program
 with interleaved Intel PT directives: