Correct a comment of GPBFieldDescriptor
The comment of property 'mapKeyDataType' has a mistake.
If the field is a map, the value's type is not fieldType.
Instead, it's dataType.
diff --git a/objectivec/GPBDescriptor.h b/objectivec/GPBDescriptor.h
index 331d444..fd66b0e 100644
--- a/objectivec/GPBDescriptor.h
+++ b/objectivec/GPBDescriptor.h
@@ -182,7 +182,7 @@
@property(nonatomic, readonly, getter=isOptional) BOOL optional;
/** Type of field (single, repeated, map). */
@property(nonatomic, readonly) GPBFieldType fieldType;
-/** Type of the key if the field is a map. The value's type is -fieldType. */
+/** Type of the key if the field is a map. The value's type is -dataType. */
@property(nonatomic, readonly) GPBDataType mapKeyDataType;
/** Whether the field is packable. */
@property(nonatomic, readonly, getter=isPackable) BOOL packable;
