[Doc] Fix overzealous sed in ContributeToDPCPP.md (#8114)

After #6507, the contribution guidelines became confusing.

Author: al42and

sycl/doc/developer/ContributeToDPCPP.md

@@ -69,7 +69,7 @@ end-to-end or SYCL-CTS tests.
 
 #### General guidelines
 
-- Use `sycl::` namespace instead of `sycl::`
+- Use `sycl::` namespace instead of `cl::sycl::`
 
 - Add a helpful comment describing what the test does at the beginning and
   other comments throughout the test as necessary.