Update text and link to change primary branch from master to main (FreeRTOS#1144)

* Update README.md

Update CMake FetchContent_Declare example to be consistent with primary branch name, i.e., master->main.

* Update CONTRIBUTING.md

update default/primary branch to be main vs master

---------

Co-authored-by: Tony Josi <[email protected]>

Author: simpsond

.github/CONTRIBUTING.md

@@ -33,7 +33,7 @@ When creating a new topic on the forums or filing an issue, please include as ma
 ## Contributing via pull request
 Contributions via pull requests are much appreciated. Before sending us a pull request, please ensure that:
 
-1. You are working against the latest source on the *master* branch.
+1. You are working against the latest source on the *main* branch.
 2. You check existing open, and recently merged, pull requests to make sure someone else hasn't addressed the problem already.
 3. You open an issue to discuss any significant work - we would hate for your time to be wasted.
 
@@ -63,7 +63,7 @@ Looking at the existing issues is a great way to find something to contribute on
 
 
 ## Licensing
-The FreeRTOS-Plus-TCP library is released under the MIT open source license, the text of which can be found [here](https://github.com/FreeRTOS/FreeRTOS-Plus-TCP/blob/master/LICENSE.md)
+The FreeRTOS-Plus-TCP library is released under the MIT open source license, the text of which can be found [here](https://github.com/FreeRTOS/FreeRTOS-Plus-TCP/blob/main/LICENSE.md)
 
 Additional license files can be found in the folders containing any supplementary libraries licensed by their respective copyright owners where applicable.
 

README.md

@@ -45,7 +45,7 @@ Add the following into your project's main or a subdirectory's `CMakeLists.txt`:
 ```cmake
 FetchContent_Declare( freertos_plus_tcp
   GIT_REPOSITORY https://github.com/FreeRTOS/FreeRTOS-Plus-TCP.git
-  GIT_TAG        master #Note: Best practice to use specific git-hash or tagged version
+  GIT_TAG        main #Note: Best practice to use specific git-hash or tagged version
   GIT_SUBMODULES "" # Don't grab any submodules since not latest
 )
 ```