Fix documentation: building.md broken link and TOC

* Vela configuarion example broken link fixed to correct file.
* Added missing entry (Building for default configuration) in the TOC

Change-Id: I7c4c9a6db742968db144a4cfcfe97a159e960268
diff --git a/docs/sections/building.md b/docs/sections/building.md
index d2e2546..2bfc1c5 100644
--- a/docs/sections/building.md
+++ b/docs/sections/building.md
@@ -7,6 +7,7 @@
     - [Preparing build environment](#preparing-build-environment)
       - [Fetching submodules](#fetching-submodules)
       - [Fetching resource files](#fetching-resource-files)
+    - [Building for default configuration](#building-for-default-configuration)
     - [Create a build directory](#create-a-build-directory)
     - [Configuring the build for MPS3 SSE-300](#configuring-the-build-for-mps3-sse-300)
       - [Using GNU Arm Embedded toolchain](#using-gnu-arm-embedded-toolchain)
@@ -677,7 +678,7 @@
   `ethos-u55-64`, and `ethos-u55-32`.
 - `--block-config-limit`: Limits the block config search space. Use `zero` for unlimited search space.
 - `--config`: Specifies the path to the Vela configuration file. The format of the file is a Python ConfigParser `.ini`
-    file. An example can be found in the `dependencies` folder [vela.ini](../../scripts/vela/vela.ini).
+    file. An example can be found in the `dependencies` folder [default_vela.ini](../../scripts/vela/default_vela.ini).
 - `--memory-mode`: Selects the memory mode to use as specified in the Vela configuration file.
 - `--system-config`: Selects the system configuration to use as specified in the Vela configuration file.