Fix typo on pull request template (#1103)

Author: kevinAlbs

docs/pull_request_template.md

@@ -1,7 +1,7 @@
 <!--- If applicable, issue number goes here, e.g. GODRIVER-ABCD -->
 
 ## Summary
-<!--- A summary of the changes propsed by this pull request. -->
+<!--- A summary of the changes proposed by this pull request. -->
 
 ## Background & Motivation
 <!--- Rationale for the pull request. -->