Use correct verb form in the pull request template

When "forget" is used with a gerund, it refers to an action that has already happened.

Author: pamil

.github/PULL_REQUEST_TEMPLATE.md

@@ -2,9 +2,9 @@
 | ------------- | ---
 | Branch?       | 3.4 or master / 2.7, 2.8 or 3.3 <!-- see comment below -->
 | Bug fix?      | yes/no
-| New feature?  | yes/no <!-- don't forget updating src/**/CHANGELOG.md files -->
+| New feature?  | yes/no <!-- don't forget to update src/**/CHANGELOG.md files -->
 | BC breaks?    | yes/no
-| Deprecations? | yes/no <!-- don't forget updating UPGRADE-*.md files -->
+| Deprecations? | yes/no <!-- don't forget to update UPGRADE-*.md files -->
 | Tests pass?   | yes/no
 | Fixed tickets | #... <!-- #-prefixed issue number(s), if any -->
 | License       | MIT