[text-auditor] Fix clear user-facing typo and grammar issues in field/module text

## Text Improvements

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

### 1. Typo in Memcached exported field description

**File:** `metricbeat/module/memcached/stats/_meta/fields.yml` (line 31)
**Current text:** `Numer of successful connect attempts to this server since it has been started.`
**Suggested fix:** `Number of successful connect attempts to this server since it has been started.`
**Why:** Clear typo (`Numer` → `Number`) in user-facing field text.

### 2. Grammar error in PostgreSQL exported field descriptions

**File:** `packetbeat/protos/pgsql/_meta/fields.yml` (lines 25 and 30)
**Current text:** `If the SELECT query if successful, this field is set to the number ...`
**Suggested fix:** `If the SELECT query is successful, this field is set to the number ...`
**Why:** Clear duplicated-word grammar error (`if successful` should be `is successful`) in user-facing field text.

### 3. Demonstrative agreement errors in module documentation source text

**Files:**
- `docs/extend/creating-metricbeat-module.md` (line 101)
- `auditbeat/module/auditd/_meta/docs.md` (line 149)
- `metricbeat/module/etcd/leader/_meta/docs.md` (line 1)
- `metricbeat/module/etcd/self/_meta/docs.md` (line 1)
- `metricbeat/module/etcd/store/_meta/docs.md` (line 1)
- `metricbeat/module/etcd/metrics/_meta/docs.md` (line 6)

**Current text examples:**
- `This steps assume you have checked out ...`
- `This files are loaded ...`
- `This metrics is being read ...`

**Suggested fixes:**
- `This steps` → `These steps`
- `This files` → `These files`
- `This metrics` → `These metrics`

**Why:** Clear singular/plural agreement errors in user-facing docs source.

### 4. Typo in dockerlogbeat Mage error text

**File:** `x-pack/dockerlogbeat/magefile.go` (line 111)
**Current text:** `error opening temp dur: %w`
**Suggested fix:** `error opening temp dir: %w`
**Why:** Clear typo (`dur` → `dir`) in user-facing command error output.

## Completeness Check

Performed targeted repo-wide searches for each problem family:
- `Numer of successful connect attempts` → 3 matches total; 1 non-generated source occurrence to fix (`metricbeat/.../_meta/fields.yml`), others are generated/reference copies.
- `If the SELECT query if successful` → 6 matches total; 2 non-generated source occurrences to fix (`packetbeat/protos/pgsql/_meta/fields.yml`), others are generated/reference copies.
- `\bThis (steps|files|metrics)\b` → 12 matches total; 6 non-generated user-facing source occurrences listed above (remaining matches are generated docs or non-user-facing comments).
- `error opening temp dur` → 1 match total; 1 true positive.

## Suggested Actions

- [ ] Fix `Numer` → `Number` in `metricbeat/module/memcached/stats/_meta/fields.yml:31`.
- [ ] Fix `If the SELECT query if successful` → `If the SELECT query is successful` in `packetbeat/protos/pgsql/_meta/fields.yml:25,30`.
- [ ] Fix `This steps/files/metrics` agreement errors in the six source docs listed above.
- [ ] Fix `temp dur` → `temp dir` in `x-pack/dockerlogbeat/magefile.go:111`.

## Duplicate Check

Checked `/tmp/previous-findings.json` and GitHub issue search for the exact phrases above; no open `[text-auditor]` issue currently tracks these specific problem families.

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