Fix typo in showWebView

[renkun-ken]

What problem did you solve?

Closes #309

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Run the following code and the webview should look like above:

library(knitr)
library(kableExtra)
dt <- mtcars[1:5, 1:4]

# HTML table
kable(dt, format = "html", caption = "Demo Table") %>%
  kable_styling(bootstrap_options = "striped",
    full_width = F) %>%
  add_header_above(c(" ", "Group 1" = 2, "Group 2[note]" = 2)) %>%
  add_footnote(c("table footnote"))

[Ikuyadeu]

Thank you!
LGTM.