docs: fix duplicate anchor so Documentation TOC link resolves

The Artifacts table of contents links to [Documentation](#documentation),
but the Documentation section reused the same <a name="source-code">
anchor as the Source Code section just above it. As a result the
Documentation TOC entry did not resolve to its section. Rename the
anchor to "documentation" to match the TOC link.

Signed-off-by: iacker <iacker@users.noreply.github.com>

Author: iacker

CONTRIBUTING.md

@@ -59,7 +59,7 @@ The GPU Operator releases follow [calendar versioning](https://calver.org/).
 
 The NVIDIA GPU Operator source code is available on GitHub at https://github.com/NVIDIA/gpu-operator
 
-### <a name="source-code"></a> Documentation
+### <a name="documentation"></a> Documentation
 
 The official NVIDIA GPU Operator documentation is available at https://docs.nvidia.com/datacenter/cloud-native/gpu-operator/latest/index.html