Add the solidity linter command to the PR template (#1653)

* Add the solidity linter command to the PR template

The PR template states that a contributor should run the
Solidity/JS linters before submission. However, it only states the
command for the JS linter.

This commit adds the Solidity linter command explicitly.

* Use past tense in the list of prerequisites

.github/PULL_REQUEST_TEMPLATE.md

@@ -16,6 +16,7 @@ Fixes #
     (https://github.com/OpenZeppelin/openzeppelin-solidity/blob/master/CONTRIBUTING.md),
   - added tests where applicable to test new functionality,
   - made sure that your contracts are well-documented,
-  - run the JS/Solidity linters and fixed any issues (`npm run lint:fix`), and
+  - run the Solidity linter (`npm run lint:sol`) and fixed any issues,
+  - run the JS linter and fixed any issues (`npm run lint:fix`), and
   - updated the changelog, if applicable.
 -->