fix several invalid code blocks in docstrings (JuliaLang#49174)

Author: Pangoraw

base/file.jl

@@ -844,7 +844,7 @@ julia> readdir("base", join=true)
  ⋮
  "base/version_git.sh"
  "base/views.jl"
- "base/weakkeydict.jl"```
+ "base/weakkeydict.jl"
 
 julia> readdir(abspath("base"), join=true)
 145-element Array{String,1}:

base/namedtuple.jl

@@ -69,6 +69,7 @@ The name-value pairs can also be provided by splatting a named tuple or any
 iterator that yields two-value collections holding each a symbol as first
 value:
 
+```jldoctest
 julia> keys = (:a, :b, :c); values = (1, 2, 3);
 
 julia> NamedTuple{keys}(values)

base/shell.jl

@@ -361,12 +361,12 @@ cmdargs = Base.shell_escape_wincmd("Passing args with %cmdargs% works 100%!")
 run(setenv(`cmd /C echo %cmdargs%`, "cmdargs" => cmdargs))
 ```
 
-!warning
+!!! warning
     The argument parsing done by CMD when calling batch files (either inside
     `.bat` files or as arguments to them) is not fully compatible with the
     output of this function. In particular, the processing of `%` is different.
 
-!important
+!!! important
     Due to a peculiar behavior of the CMD parser/interpreter, each command
     after a literal `|` character (indicating a command pipeline) must have
     `shell_escape_wincmd` applied twice since it will be parsed twice by CMD.

stdlib/LinearAlgebra/src/svd.jl

@@ -213,7 +213,6 @@ Base.propertynames(F::SVD, private::Bool=false) =
 
 Return the singular values of `A`, saving space by overwriting the input.
 See also [`svdvals`](@ref) and [`svd`](@ref).
-```
 """
 svdvals!(A::StridedMatrix{T}) where {T<:BlasFloat} = isempty(A) ? zeros(real(T), 0) : LAPACK.gesdd!('N', A)[2]
 svdvals!(A::StridedVector{T}) where {T<:BlasFloat} = svdvals!(reshape(A, (length(A), 1)))

stdlib/REPL/src/REPL.jl

@@ -3,13 +3,14 @@
 """
 Run Evaluate Print Loop (REPL)
 
-    Example minimal code
-    ```
-    import REPL
-    term = REPL.Terminals.TTYTerminal("dumb", stdin, stdout, stderr)
-    repl = REPL.LineEditREPL(term, true)
-    REPL.run_repl(repl)
-    ```
+Example minimal code
+
+```julia
+import REPL
+term = REPL.Terminals.TTYTerminal("dumb", stdin, stdout, stderr)
+repl = REPL.LineEditREPL(term, true)
+REPL.run_repl(repl)
+```
 """
 module REPL