for loop tuple syntax not rendered correctly


### Example
```nim
runnableExamples:
  var data = @["one", "two"]
  for (i, d) in data.pairs:
    echo d
```

### Current Output
Syntax error
the temp file contains:
```nim
var data = @["one", "two"]
for (i) = in data.pairs:
  echo d
```

### Expected Output
nim doc compile and run correctly

```
$ nim -v
Nim Compiler Version 1.0.2
```
