Replace -Zno-parallel-llvm with -Zno-parallel-backend

This was renamed by rust-lang/rust#116791.

Author: TimNN

src/backend/debugging.md

@@ -208,7 +208,7 @@ tutorial above):
 - The `-Z print-llvm-passes` option will print out LLVM optimization passes being run
 - The `-Z time-llvm-passes` option measures the time of each LLVM pass
 - The `-Z verify-llvm-ir` option will verify the LLVM IR for correctness
-- The `-Z no-parallel-llvm` will disable parallel compilation of distinct compilation units
+- The `-Z no-parallel-backend` will disable parallel compilation of distinct compilation units
 - The `-Z llvm-time-trace` option will output a Chrome profiler compatible JSON file
   which contains details and timings for LLVM passes.
 - The `-C llvm-args=-opt-bisect-limit=<index>` option allows for bisecting LLVM