doc: fix old master branch references to main (#10458)

Author: Jefffrey

README.md

@@ -51,7 +51,7 @@ Here are links to some important information
 - [Rust Getting Started](https://datafusion.apache.org/user-guide/example-usage.html)
 - [Rust DataFrame API](https://datafusion.apache.org/user-guide/dataframe.html)
 - [Rust API docs](https://docs.rs/datafusion/latest/datafusion)
-- [Rust Examples](https://github.com/apache/datafusion/tree/master/datafusion-examples)
+- [Rust Examples](https://github.com/apache/datafusion/tree/main/datafusion-examples)
 - [Python DataFrame API](https://arrow.apache.org/datafusion-python/)
 - [Architecture](https://docs.rs/datafusion/latest/datafusion/index.html#architecture)
 

docs/source/index.rst

@@ -46,7 +46,7 @@ The `example usage`_ section in the user guide and the `datafusion-examples`_ co
 Please see the `developer’s guide`_ for contributing and `communication`_ for getting in touch with us.
 
 .. _example usage: user-guide/example-usage.html
-.. _datafusion-examples: https://github.com/apache/datafusion/tree/master/datafusion-examples
+.. _datafusion-examples: https://github.com/apache/datafusion/tree/main/datafusion-examples
 .. _developer’s guide: contributor-guide/index.html#developer-s-guide
 .. _communication: contributor-guide/communication.html