[text-auditor] Fix AWS autodiscover EC2/ELB field-description text errors

## Text Improvements

The following user-facing text issues were found in the repository. Each is a low-effort fix.

### 1. EC2 autodiscover field docs incorrectly describe instances as "listeners"

**File:** `x-pack/libbeat/autodiscover/providers/aws/ec2/_meta/fields.yml` (lines 2, 4, 12)
**Current text:**
- `title: "EC2 Listener"`
- `AWS EC2 Listeners`
- `Represents an AWS EC2 Listener, e.g. state of an EC2.`

**Suggested fix:**
- `title: "EC2 Instance"`
- `AWS EC2 Instances`
- `Represents an AWS EC2 instance, e.g. state of an EC2 instance.`

**Why:** This section documents `aws.ec2` fields (instance architecture, image, subnet, VPC), so referring to EC2 resources as "listeners" is a direct terminology mismatch and can mislead users.

### 2. ELB field description is truncated

**File:** `x-pack/libbeat/autodiscover/providers/aws/elb/_meta/fields.yml` (line 45)
**Current text:** `List of security groups for this`
**Suggested fix:** `List of security groups for this ELB.`
**Why:** The sentence is incomplete and leaves the described object unspecified.

## Suggested Actions

- [ ] Replace the EC2 "Listener" wording family in `x-pack/libbeat/autodiscover/providers/aws/ec2/_meta/fields.yml` with "Instance" wording.
- [ ] Complete the truncated `security_groups` description in `x-pack/libbeat/autodiscover/providers/aws/elb/_meta/fields.yml`.

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> - [x] expires on Sep 17, 2026, 1:24 PM UTC

Start with x-pack/libbeat/autodiscover/providers/aws/ec2/_meta/fields.yml at the listed lines and x-pack/libbeat/autodiscover/providers/aws/elb/_meta/fields.yml around line 45. Update the EC2 terminology to describe instances and complete the ELB security-groups sentence; done means both field descriptions match the suggested wording and no "Listener" mismatch remains.

Written by the indexing model from the issue text.