doc: fix docstring for Dates.format

[inkydragon]

Close #35286

Current docstring:

julia> using Dates

help?> Dates.format

...

  as a literal you can use the escape character backslash. The string
  "1996y01m" can be produced with the format "yyyy\ymm\m".

julia>

julia> using Dates

julia> "yyyy\ymm\m"
ERROR: ParseError:
# Error @ REPL[36]:1:6
"yyyy\ymm\m"
#    └┘ ── invalid escape sequence
Stacktrace:
 [1] top-level scope
   @ REPL:1

julia> Dates.format(Dates.now(), raw"yyyy\ymm\m")
"2024y11m"

julia> Dates.format(Dates.now(), "yyyy\\ymm\\m")  # Another way
"2024y11m"