README: add missing verbatim wrap

Add a missing verbatim wrap to make the "diagram" look more as-intended.

Change-Id: I3f6b8c430a3b3833356e5c235c3752bf3042e5cb
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/README.md b/README.md
index 8a6019c..26f2a10 100644
--- a/README.md
+++ b/README.md
@@ -330,6 +330,7 @@
 One defines a series of sensors and a series of PIDs that utilize those sensors
 as inputs (and outputs).
 
+```
  The thermal PID
 
  set-point -------->|---|---> RPM
@@ -346,6 +347,7 @@
  thermal-out ----->|     |-----> RPM
  thermal-out ----->| MAX |
  thermal-out ----->|_____|
+```
 
 ## Notes