Merge pull request #130 from Rayne/docs-custom-typo

Fixed typo in writing-validators.md

Author: akrabat

doc/book/writing-validators.md

@@ -76,7 +76,7 @@ message template.
 The following example demonstrates a more complex set of validation rules:
 
 - The input must be numeric.
-- The input must fall within a range of bouncary values.
+- The input must fall within a range of boundary values.
 
 An input value would fail validation for exactly one of the following reasons: