Fix english error message gramma

Fix #606

include/rapidjson/error/en.h

diff --git a/include/rapidjson/error/en.h b/include/rapidjson/error/en.h
index c2315fd..2db838b 100644
--- a/include/rapidjson/error/en.h
+++ b/include/rapidjson/error/en.h
@@ -38,7 +38,7 @@
         case kParseErrorNone:                           return RAPIDJSON_ERROR_STRING("No error.");
 
         case kParseErrorDocumentEmpty:                  return RAPIDJSON_ERROR_STRING("The document is empty.");
-        case kParseErrorDocumentRootNotSingular:        return RAPIDJSON_ERROR_STRING("The document root must not follow by other values.");
+        case kParseErrorDocumentRootNotSingular:        return RAPIDJSON_ERROR_STRING("The document root must not be followed by other values.");
     
         case kParseErrorValueInvalid:                   return RAPIDJSON_ERROR_STRING("Invalid value.");