Update PULL_REQUEST_TEMPLATE.md (forem#791) [ci skip]

On macOS, and probably on other operating systems, double clicking a word, selects it. The OS sees underscores as word characters, which isn't the case for hyphens, so that this_word is seen as one word, whereas this-word isn't. Replacing the dashes with underscores makes these placeholders easier to select and replace.

Author: frederikcreemers

.github/PULL_REQUEST_TEMPLATE.md

@@ -15,4 +15,4 @@
   - [ ] no documentation needed
 
 ## [optional] What gif best describes this PR or how it makes you feel?
-![alt-text](gif-link)
+![alt_text](gif_link)