A small nitpick and a redundant method call in func parseStat()

[vak-rashu]

A small nitpick:

At line 181, https://github.com/prometheus/procfs/blob/master/stat.go#L181, the comment is passed as:
// parseStat parses the metrics from /proc/[pid]/stat.

however, parseStat parses the system-wide /proc/stat file rather than /proc/[pid]/stat, so the comment appears to be inaccurate.

Repetitive method calling:
In the same function:

buf := make([]byte, 0, 8*1024)
	scanner.Buffer(buf, 1024*1024)

	for scanner.Scan() {
		line := scanner.Text()
		parts := strings.Fields(scanner.Text())

(Line https://github.com/prometheus/procfs/blob/master/stat.go#L197)
Since line already contains the current scanner text, this could instead be:
parts := strings.Fields(line)

They both are very small issues but worth noting.

If accepted I would like to make a PR for these changes.