docs: fix "eg." to "e.g." in cloudwatch comments and README (#37404)

### Issue

N/A (self-discovered)

### Reason for this change

The abbreviation "e.g." (Latin: *exempli gratia*) should include periods. The codebase consistently uses "e.g." but has a few instances of "eg." without periods.

### Description of changes

- `aws-cloudwatch/lib/dashboard.ts`: "eg." → "e.g." in JSDoc comment
- `aws-cloudwatch/README.md`: "eg." → "e.g."

### Description of how you validated changes

Comment and documentation-only changes. No functional impact.

### Checklist

- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

Author: syukawa-gh

packages/aws-cdk-lib/aws-cloudwatch/README.md

@@ -959,7 +959,7 @@ You can add a widget after object instantiation with the method
 ### Interval duration for dashboard
 
 Interval duration for metrics in dashboard. You can specify `defaultInterval` with
-the relative time(eg. 7 days) as `Duration.days(7)`.
+the relative time (e.g. 7 days) as `Duration.days(7)`.
 
 ```ts
 import * as cw from 'aws-cdk-lib/aws-cloudwatch';

packages/aws-cdk-lib/aws-cloudwatch/lib/dashboard.ts

@@ -39,7 +39,7 @@ export interface DashboardProps {
 
   /**
    * Interval duration for metrics.
-   * You can specify defaultInterval with the relative time(eg. cdk.Duration.days(7)).
+   * You can specify defaultInterval with the relative time (e.g. cdk.Duration.days(7)).
    *
    * Both properties `defaultInterval` and `start` cannot be set at once.
    *