Fix typo

contributing.md

@@ -8,5 +8,5 @@ Before add your tip take in consideration the following guidelines:
 - Inside the folder, each file is a tip.
 - If your tip is very similar to a tip that already exists, consider extend the first tip.
 - Don't add more than one tip per file.
-- The text that describe the tip have to be concesive and precise.
+- The text that describe the tip have to be concessive and precise.
 - For avoid extend the tip, you can reference other tips of the site.