Update contributing guidelines for commit message format (#189922)

This style of putting code terms in backticks is implied not stated. 

https://github.com/flutter/packages/pull/12145/changes#r3625889893

docs/ecosystem/contributing/README.md

diff --git a/docs/ecosystem/contributing/README.md b/docs/ecosystem/contributing/README.md
index 3fdd223..165248a 100644
--- a/docs/ecosystem/contributing/README.md
+++ b/docs/ecosystem/contributing/README.md
@@ -25,6 +25,7 @@
 - Entries should end with a `.`.
 - Breaking changes should be introduced with `**BREAKING CHANGE**:`, or `**BREAKING CHANGES**:` if there is a sub-list of changes.
   - Breaking change notifications should include information about how to migrate. If extensive migration is required, this can be a reference to a longer description elsewhere (usually README.md) rather than inline instructions.
+- Code references should be enclosed in backticks.
 
 Example:
 ```