[doc] add improvement tag for PR title in PR template (#15)

Author: golanglibs

pull_request_template.md

@@ -3,6 +3,6 @@
 ## Issue Link
 
 ## Checklist
-- [ ] I have prefixed the title of this PR with one of the following: [feature] [bugfix] [minor] [refactor] [doc]
+- [ ] I have prefixed the title of this PR with one of the following: [feature] [bugfix] [improvement] [minor] [refactor] [doc]
 - [ ] If the PR is a feature, bugfix, minor or refactor, I have added thorough tests
 - [ ] I have updated documentation both in code and README.md