Fix code blocks in NEWS.md (#7518)

Add a missing triple-backtick separator.

Separate the indented code blocks from the preceding paragraph because
otherwise Pandoc fails to realise that the whitespace followed by
triple-backtick denotes a fenced code block.

Author: aitap

NEWS.md

@@ -26,6 +26,7 @@
 2. `melt()` returns an integer column for `variable` when `measure.vars` is a list of length=1, consistent with the documented behavior, [#5209](https://github.com/Rdatatable/data.table/issues/5209). Thanks to @tdhock for reporting. Any users who were relying on this behavior can change `measure.vars=list("col_name")` (output `variable` was column name, now is column index/integer) to `measure.vars="col_name"` (`variable` still is column name). This change has been planned since 1.16.0 (25 Aug 2024).
 
 3. Rolling functions `frollmean` and `frollsum` distinguish `Inf`/`-Inf` from `NA` to match the same rules as base R when `algo="fast"` (previously they were considered the same). If your input into those functions has `Inf` or `-Inf` then you will be affected by this change. As a result, the argument that controls the handling of `NA`s has been renamed from `hasNA` to `has.nf` (_has non-finite_). `hasNA` continues to work with a warning, for now.
+
     ```r
     ## before
     frollsum(c(1,2,3,Inf,5,6), 2)
@@ -34,8 +35,10 @@
     ## now
     frollsum(c(1,2,3,Inf,5,6), 2)
     #[1]  NA   3   5 Inf Inf  11
+    ```
 
 4. `frollapply` result is not coerced to numeric anymore. Users' code could possibly break if it depends on forced coercion of input/output to numeric type.
+
     ```r
     ## before
     frollapply(c(F,T,F,F,F,T), 2, any)
@@ -45,6 +48,7 @@
     frollapply(c(F,T,F,F,F,T), 2, any)
     #[1]    NA  TRUE  TRUE FALSE FALSE  TRUE
     ```
+
     Additionally argument names in `frollapply` has been renamed from `x` to `X` and `n` to `N` to avoid conflicts with common argument names that may be passed to `...`, aligning to base R API of `lapply`. `x` and `n` continue to work with a warning, for now.
 
 5. Negative and missing values of `n` argument of adaptive rolling functions trigger an error.
@@ -233,6 +237,7 @@ See [#2611](https://github.com/Rdatatable/data.table/issues/2611) for details. T
     ```
 
 18. New helper `frolladapt` to facilitate applying rolling functions over windows of fixed calendar-time width in irregularly-spaced data sets, thereby bypassing the need to "augment" such data with placeholder rows, [#3241](https://github.com/Rdatatable/data.table/issues/3241). Thanks to @jangorecki for implementation.
+
     ```r
     idx = as.Date("2025-09-05") + c(0,4,7,8,9,10,12,13,17)
     dt = data.table(index=idx, value=seq_along(idx))