Fix typo

docs/release_guide.pod

@@ -138,7 +138,7 @@ You can get an automatically generated list by running
   $ ./perl6 tools/contributors.pl6
 
 To obtain all contributors, ensure you have all supporting repositories
-checked out, before running (this is can be achieved by building rakudo
+checked out, before running (this can be achieved by building rakudo
 and running a spectest). Please check the result manually for duplicates
 and other errors. Note that you may not be able to run your system perl6
 in a local checkout, you may have to wait until you build in this