Indent pandoc args correctly relative to the key

From Pandoc 2.2.2, HsYAML is the new YAML parser and it has a stricter
interpretation of YAML 1.2. My previous indentation was causing an
error that resulted, I believe, in the pandoc_args being ignored (?).

I detected this via a test failure re: standard output. The parsing
error was showing up on standard output in a case where standard output
should have been empty.

https://stackoverflow.com/questions/31839686/wrapping-a-list-over-multiple-lines-yaml

jgm/pandoc#4959 (comment)

NEWS.md

@@ -1,5 +1,7 @@
 # reprex *development version*
 
+* The YAML of reprex's template has been updated in light of the stricter YAML parser used in Pandoc >= 2.2.2.
+
 * `rlang::set_attrs()` has been soft-deprecated and is no longer used internally.
 
 # reprex 0.2.1

R/whisker.R

@@ -56,18 +56,18 @@ yaml_md <- function(flavor = c("gfm", "md"),
     "    pandoc_args: [",
     if (flavor == "gfm") {
       c(
-    "      '-f', 'markdown-implicit_figures',",
-    "      '-t', 'commonmark',"
+    "                 '-f', 'markdown-implicit_figures',",
+    "                 '-t', 'commonmark',"
       )
     },
     if (!is.null(pandoc_version)) {
       if (pandoc_version < "1.16") {
-    "      --no-wrap"
+    "                 --no-wrap"
       } else {
-    "      --wrap=preserve"
+    "                 --wrap=preserve"
       }
     },
-    "    ]",
+    "                 ]",
     "---"
   )
   ## prepend with `#' ` in a separate step because