Fix spelling error in CONTRIBUTING.md (sorin-ionescu#1590)

Author: apjanke

CONTRIBUTING.md

@@ -32,7 +32,7 @@ there are a number of additional things to keep in mind.
    - Prefer `zstyle` over environment variables for configuration.
    - Prefer (( ... )) over [[ ... ]] for arithmetic expression.
    - Use the function keyword to define functions.
-   - The 80 character hard limit can be waved for readability.
+   - The 80 character hard limit can be waived for readability.
 
 #### Using an Alternative zprezto Directory