Fix typo on DescriptorArray class comment

Bug: 
Change-Id: I236e6efe4142741ae8dc7e4ca4acdf8a65890c16
Reviewed-on: https://chromium-review.googlesource.com/647534Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47915}

src/objects/descriptor-array.h

@@ -40,7 +40,7 @@ class EnumCache : public Tuple2 {
 //   [1]: Enum cache.
 //   [2]: first key (and internalized String)
 //   [3]: first descriptor details (see PropertyDetails)
-//   [4]: first value for constants | Smi(1) when not usedA
+//   [4]: first value for constants | Smi(1) when not used
 //
 //   [2 + number of descriptors * 3]: start of slack
 class DescriptorArray : public FixedArray {