Update proposals/common-definitions/README.md

Co-authored-by: danielpeintner <[email protected]>

Author: egekorkan

proposals/common-definitions/README.md

@@ -146,7 +146,7 @@ These set a default for the whole Thing. The mechanism is the same for all the t
 
 Note: Even if a single form of an affordance is not complete, a defaultable element should exist.
 
-### Elementss
+### Elements
 
 #### Security