metal: Fix stray '\' prefix on MSL shaders

Seems harmless, but I'm assuming it was not intentional.

Change-Id: I86875f8bbe5923435b4c02cd224197ec2dc6f3a1
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/60924
Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Austin Eng <enga@chromium.org>
Auto-Submit: Ben Clayton <bclayton@google.com>
Commit-Queue: Austin Eng <enga@chromium.org>

src/dawn_native/metal/ShaderModuleMTL.mm

@@ -317,7 +317,7 @@ namespace dawn_native { namespace metal {
         // category. -Wunused-variable in particular comes up a lot in generated code, and some
         // (old?) Metal drivers accidentally treat it as a MTLLibraryErrorCompileError instead
         // of a warning.
-        msl = R"(\
+        msl = R"(
 #ifdef __clang__
 #pragma clang diagnostic ignored "-Wall"
 #endif