fix error message for size_mismatch

Author: grisumbras

include/boost/json/error.hpp

@@ -142,7 +142,7 @@ enum class error
     /// `double` was expected during conversion
     not_double,
 
-    /// JSON array has size incompatible with target
+    /// source composite has size incompatible with target
     size_mismatch,
 
     /// none of the possible conversions were successful

include/boost/json/impl/error.ipp

@@ -81,7 +81,7 @@ case error::not_string: return "value is not a string";
 case error::not_int64: return "value is not a std::int64_t number";
 case error::not_uint64: return "value is not a std::uint64_t number";
 case error::not_double: return "value is not a double";
-case error::size_mismatch: return "array size does not match target size";
+case error::size_mismatch: return "source composite size does not match target size";
 case error::exhausted_variants: return "exhausted all variants";
 case error::unknown_name: return "unknown name";
     }