fixes rst parsing Markdown CodeblockFields blocking the loop (#24128)

```nim
import packages/docutils/[rst, rstgen]

let message = """```llvm-profdata"""

echo rstgen.rstToHtml(message, {roSupportMarkdown}, nil)
```

lib/packages/docutils/rst.nim

@@ -1526,7 +1526,7 @@ proc parseMarkdownCodeblockFields(p: var RstParser): PRstNode =
     result = nil
   else:
     result = newRstNode(rnFieldList)
-  while currentTok(p).kind != tkIndent:
+  while currentTok(p).kind notin {tkIndent, tkEof}:
     if currentTok(p).kind == tkWhite:
       inc p.idx
     else: