Fix small grammatical error (#2533)

R/render.R

@@ -801,7 +801,7 @@ render <- function(input,
         stop2("Functions that produce HTML output found in document targeting ",
              pandoc_to, " output.\nPlease change the output type ",
              "of this document to HTML.\n",
-             "If your aiming to have some HTML widgets shown in non-HTML format as a screenshot,\n",
+             "If you're aiming to have some HTML widgets shown in non-HTML format as a screenshot,\n",
              "please install webshot or webshot2 R package for knitr to do the screenshot, and configure it by looking at its documentation.\n",
              "Alternatively, you can allow HTML output in non-HTML formats\n",
              "by adding this option to the YAML front-matter of\nyour rmarkdown file:\n\n",