docs: fix paths example under “Restricting a Metric to Specific Log Files”

[madalinpopa]

This PR corrects a documentation example that showed paths with a single scalar string. In the grok_exporter config schema, paths must be a YAML list ([]string). For a single glob, either use path (single string) or keep paths as a one‑item list. The current example can mislead users and cause YAML/schema validation errors at startup.

Problem

The docs currently show:

- type: counter
  name: alice_occurrences_total
  help: number of log lines containing alice
  match: 'alice'
  paths: /tmp/example/*.log      # <-- scalar used for `paths` (wrong type)
  labels:
    logfile: '{{base .logfile}}'

• paths is defined in the config as a list of strings (see the PathsAndGlobs struct: Path string, Paths []string).
• Using a scalar under paths can yield errors like cannot unmarshal !!str into []string or simply fail validation, depending on the parser/version.
• Elsewhere in the same document, paths is shown as a list, which makes this example inconsistent.