snippets: fix wrong function in md snippets

- Replace `get_date` by `get_datetime` in the frontmatter snippets for
  markdown.

Co-Authored-by: iGor milhit <igor.milhit@unige.ch>

my_snippets/markdown.lua

@@ -97,7 +97,7 @@ return {
       {
         i(1, "title"),
         i(2, "tags"),
-        f(get_date, {}),
+        f(get_datetime, {}),
         f(get_id, {})
       }
     )
@@ -121,7 +121,7 @@ return {
       ]],
       {
         f(get_month_year, {}),
-        f(get_date, {}),
+        f(get_datetime, {}),
         f(get_month, {})
       }
     )