doc: fix broken :ref: link

Typo in :ref:`hello_world`  (regular quote vs. back tick)

Change-Id: I77853f85b9c71751307ef105b6babcb0cfbc9060
Signed-off-by: David B. Kinder <david.b.kinder@intel.com>
diff --git a/boards/arm/nucleo_l476rg/doc/nucleol476rg.rst b/boards/arm/nucleo_l476rg/doc/nucleol476rg.rst
index 9ffe830..3eb94a8 100644
--- a/boards/arm/nucleo_l476rg/doc/nucleol476rg.rst
+++ b/boards/arm/nucleo_l476rg/doc/nucleol476rg.rst
@@ -184,7 +184,7 @@
 Flashing an application to Nucleo L476RG
 ----------------------------------------
 
-The sample application :ref:'hello_world` is being used in this tutorial:
+The sample application :ref:`hello_world` is being used in this tutorial:
 
 To build the Zephyr kernel and application, enter: