Replace zero-width whitespace with a visible \

People tried to copy the `r?` which had an invisible zero-width
whitespace, causing rustbot to not recognize the instruction, and which
makes it difficult to figure out why it didn't work.

Author: jieyouxu

.github/pull_request_template.md

@@ -7,6 +7,6 @@ tracking issue or there are none, feel free to ignore this.
 This PR will get automatically assigned to a reviewer. In case you would like
 a specific user to review your work, you can assign it to them by using
 
-    r​? <reviewer name>
+    r\? <reviewer name> (with the `\` removed)
 -->
 <!-- homu-ignore:end -->