chore(docs): Fix proofreading mistake in v0.35.0 upgrade guide (#19551)

Suggested by #19528 (review)

Signed-off-by: Jesse Szwedko <[email protected]>

website/content/en/highlights/2023-12-19-0-35-0-upgrade-guide.md

@@ -65,8 +65,8 @@ The `requests_completed_total`, `request_duration_seconds`, and `requests_receiv
 
 Vector has the ability to [interpolate environment variables into its
 configuration](/docs/reference/configuration/#environment-variables) as a means of templating
-configuration. Currently, if an environment variable is undefined (unless the `?` interpolation
-syntax is used) Vector outputs a warning and replaces the environment variable with an empty string.
+configuration. Currently, if an environment variable is undefined, unless the `?` interpolation
+syntax is used, Vector outputs a warning and replaces the environment variable with an empty string.
 In a future release , this will change from a warning to a hard error. This behavior can be opted
 into earlier by using the `--strict-env-vars` flag (or `VECTOR_STRICT_ENV_VARS` environment
 variable) when running Vector.