Add a missing space in the error message of SetIndexBuffer

BUG=dawn:887

Change-Id: I108377328ce735a2daaca9512ded6b35e0bfaa14
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/74460
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/dawn_native/RenderEncoderBase.cpp

@@ -281,7 +281,7 @@ namespace dawn_native {
                                     "Index format must be specified");
 
                     DAWN_INVALID_IF(offset % uint64_t(IndexFormatSize(format)) != 0,
-                                    "Index buffer offset (%u) is not a multiple of the size (%u)"
+                                    "Index buffer offset (%u) is not a multiple of the size (%u) "
                                     "of %s.",
                                     offset, IndexFormatSize(format), format);