docs: change section title

kenjis · kenjis · commit abada3c1b150 · 2023-04-12T13:42:55.000+09:00
For consistency.
diff --git a/user_guide_src/source/libraries/validation.rst b/user_guide_src/source/libraries/validation.rst
@@ -338,8 +338,8 @@ For example, data returned by multi select dropdown:
 
 .. literalinclude:: validation/011.php
 
-Validate 1 Value
-================
+Validating 1 Value
+==================
 
 Validate one value against a rule:
 
