Fix apparent typo for 0-component swizzles.

Noticed during code read-through; unaware of any actual issues caused by
this.

Change-Id: I455e7fbc3a1d83f6319aaed1518d382ec231c8be
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/395836
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>

src/gpu/GrSwizzle.h

@@ -103,7 +103,7 @@ constexpr float GrSwizzle::ComponentIndexToFloat(std::array<float, 4> color, int
         return 1.0f;
     }
     if (idx == CToI('0')) {
-        return 1.0f;
+        return 0.0f;
     }
     SkUNREACHABLE;
 }