Add alt text for autocomplete image in strings chapter. (#1423)

strings.qmd

@@ -38,6 +38,7 @@ This is particularly useful if you use RStudio because typing `str_` will trigge
 
 ```{r}
 #| echo: false
+#| fig-alt: "`str_c` typed into the RStudio console with the autocomplete tooltip shown on top, which lists functions beginning with `str_c`. The funtion signature and beginning of the man page for the highlighted function from the autocomplete list are shown in a panel to its right."
 
 knitr::include_graphics("screenshots/stringr-autocomplete.png")
 ```