Fix Vale issues for replication_and_failover_external.md

The following issues occurred when we ran a linting tool called "Vale" against this Markdown page in the GitLab documentation. The first number on each line indicates the line number where the error occurred.

To fix the issues, open a merge request for the markdown file, which you can find in the /doc directory, under /administration/redis/replication_and_failover_external.md. When you open the MR, a bot will automatically ping the appropriate GitLab technical writer. They will work with you to merge the changes.

If you feel any of these warnings are incorrect, please create an issue with the tw-test label so we can improve our Vale tests. Thanks!

Please replace future tense with present tense on the following lines:

| 24:62 - FutureTenseAvoid using future tense "will work"    |
| 56:70 - FutureTenseAvoid using future tense "will take"    |
| 252:34 - FutureTenseAvoid using future tense "will be"     |
| 264:40 - FutureTenseAvoid using future tense "will be"     |
| 265:1 - FutureTenseAvoid using future tense "will change"  |
| 268:54 - FutureTenseAvoid using future tense "will be"     |
| 268:16 - FutureTenseAvoid using future tense "will happen" |