That image is actually HTML you can copy and paste into your word doc.
Borrowing from Rich's comment on a related github issue,
The kableExtra package recommended (01-2018) that the way to get output tables into a Word document is to copy/paste HTML table output from the Viewer directly into an open Word doc.
Another option is to just use the rtf_output format that opens in Word (as an RTF file, which could then be saved as a .docx file, for instance).
The knitr::kable() function is able to produce native table code for Word documents, and, the pander package also has this capability (with more options for table output). Here is a document that describes how: Happy collaboration with Rmd to docx.