Fix typo: "uknown" -> "unknown" (#3854)

Leon · bcardiff · commit 26402dd58c03 · 2017-01-12T11:43:33.000-03:00
diff --git a/src/json/lexer.cr b/src/json/lexer.cr
@@ -199,7 +199,7 @@ abstract class JSON::Lexer
         hexnum1.chr
       end
     else
-      raise "uknown escape char: #{char}"
+      raise "unknown escape char: #{char}"
     end
   end
 
