[backport] Docs: Fix broken code-block (#14749)

This commit indents the contents of a `code-block` in `httpclient.nim`
so that it displays correctly. The bug was introduced by 42a64245f8.

I did a quick search for other `code-block`s that are broken in the same
way, but the only other one I found (in `pegs.nim`) is not included in
the generated documentation.

lib/pure/httpclient.nim

@@ -25,13 +25,13 @@
 ## ``AsyncHttpClient``:
 ##
 ## .. code-block:: Nim
-## import asyncdispatch, httpclient
+##   import asyncdispatch, httpclient
 ##
-## proc asyncProc(): Future[string] {.async.} =
-##   var client = newAsyncHttpClient()
-##   return await client.getContent("http://example.com")
+##   proc asyncProc(): Future[string] {.async.} =
+##     var client = newAsyncHttpClient()
+##     return await client.getContent("http://example.com")
 ##
-## echo waitFor asyncProc()
+##   echo waitFor asyncProc()
 ##
 ## The functionality implemented by ``HttpClient`` and ``AsyncHttpClient``
 ## is the same, so you can use whichever one suits you best in the examples

lib/pure/pegs.nim

@@ -870,26 +870,26 @@ macro mkHandlerTplts(handlers: untyped): untyped =
   # The AST structure of *handlers[0]*:
   #
   # .. code-block::
-  # StmtList
-  #   Call
-  #     Ident "pkNonTerminal"
-  #     StmtList
-  #       Call
-  #         Ident "enter"
-  #         StmtList
-  #           <handler code block>
-  #       Call
-  #         Ident "leave"
-  #         StmtList
-  #           <handler code block>
-  #   Call
-  #     Ident "pkChar"
-  #     StmtList
-  #       Call
-  #         Ident "leave"
-  #         StmtList
-  #           <handler code block>
-  #   ...
+  #   StmtList
+  #     Call
+  #       Ident "pkNonTerminal"
+  #       StmtList
+  #         Call
+  #           Ident "enter"
+  #           StmtList
+  #             <handler code block>
+  #         Call
+  #           Ident "leave"
+  #           StmtList
+  #             <handler code block>
+  #     Call
+  #       Ident "pkChar"
+  #       StmtList
+  #         Call
+  #           Ident "leave"
+  #           StmtList
+  #             <handler code block>
+  #     ...
   proc mkEnter(hdName, body: NimNode): NimNode =
     template helper(hdName, body) {.dirty.} =
       template hdName(s, p, start) =