Fix spacing for lintrunner instructions on contributing guide (#398)

Summary:
Pull Request resolved: #398

tsia

Reviewed By: s4ayub

Differential Revision: D36915880

fbshipit-source-id: f962f9002eecc1f9a9f4a98fac5aa57ac2d45584

Author: joshuadeng

CONTRIBUTING.md

@@ -10,20 +10,20 @@ We actively welcome your pull requests.
 3. If you've changed APIs, update the documentation.
 4. Ensure the test suite passes.
 5. Make sure your code lints. You can use [lintrunner](https://github.com/pytorch/pytorch/wiki/lintrunner) to do so.
- 1. To set up:
+    1. To set up:
     ```
     pip install lintrunner
     lintrunner init
     ```
- 2. To lint your local changes:
+    2. To lint your local changes:
     ```
     lintrunner
     ```
- 3. To format locally changed files:
+    3. To format locally changed files:
     ```
     lintrunner f
     ```
-  4. To lint all files:
+    4. To lint all files:
     ```
     lintrunner --all-files
     ```