update slack workspace link

Author: James Strawson

README.md

@@ -6,4 +6,4 @@ This package contains the C library and example/testing programs for the Robot C
 
 Full API documentation, instruction manual, and examples at <http://strawsondesign.com/docs/librobotcontrol/>.
 
-We encourage questions and discussion in the [Slack Workspace](https://join.slack.com/t/librobotcontrol/shared_invite/enQtMzk0NjkxNjk0MTE0LTliNjFmMjZiZTQxOWZmNWI3NTJkN2UwOTdjNDk2OWQwMDgxNDEyMDNkMzBkODE5ODZiMWJkYmM1M2UwOTczNmE).
+We encourage questions and discussion in the [Slack Workspace](https://join.slack.com/t/librobotcontrol/shared_invite/enQtNDIxNTAzNjMxMzYzLWU1OTI5MWFhOWY5ZTM4N2FiMDdlYTY5ZmU0YjYzNzJmZjk4MmU0ZGIzNmFiNmQ5MzI3NWZkOTFiYzMxMDkwZWU).

docs/src/DoxygenLayout.xml

@@ -39,7 +39,7 @@
 
     <tab type="user" url="https://github.com/StrawsonDesign/librobotcontrol" title="GitHub"/>
     <tab type="user" url="https://github.com/StrawsonDesign/librobotcontrol/issues" title="Issue Tracker"/>
-   <tab type="user" url="https://join.slack.com/t/librobotcontrol/shared_invite/enQtMzk0NjkxNjk0MTE0LTliNjFmMjZiZTQxOWZmNWI3NTJkN2UwOTdjNDk2OWQwMDgxNDEyMDNkMzBkODE5ODZiMWJkYmM1M2UwOTczNmE" title="Slack Discussion"/>
+   <tab type="user" url="https://join.slack.com/t/librobotcontrol/shared_invite/enQtNDIxNTAzNjMxMzYzLWU1OTI5MWFhOWY5ZTM4N2FiMDdlYTY5ZmU0YjYzNzJmZjk4MmU0ZGIzNmFiNmQ5MzI3NWZkOTFiYzMxMDkwZWU" title="Slack Discussion"/>
 
   </navindex>