Update walkdir / walkpath in API table

[kescobo]

walkpath does not exist in Base, and walkdir has a different behavior, so I wasn't sure if I should make this two rows or treat them as equivalent.

Edit: this is the 1-row version. The two row version I'd propose would be

walkdir | N/A
N/A     | walkpath

examples:

julia> p = "docs"
"docs"

julia> for item in walkdir(p)
           @show item
       end
item = ("docs", ["src"], ["Project.toml", "make.jl"])
item = ("docs/src", String[], ["index.md"])

julia> for item in walkpath(p)
           @show item
       end
ERROR: UndefVarError: walkpath not defined
Stacktrace:
 [1] top-level scope
   @ ./REPL[3]:1

julia> using FilePaths

julia> for item in walkpath(p)
           @show item
       end
ERROR: MethodError: no method matching walkpath(::String)
Closest candidates are:
  walkpath(::P; topdown, follow_symlinks, onerror) where P<:AbstractPath at /home/kevin/.julia/packages/FilePathsBase/YFK4h/src/pat
h.jl:709
Stacktrace:
 [1] top-level scope
   @ ./REPL[5]:1

julia> for item in walkpath(Path(p))
           @show item
       end
item = p"docs/Project.toml"
item = p"docs/make.jl"
item = p"docs/src"
item = p"docs/src/index.md"