doc: typo for Assertions type

Signed-off-by: Frederic BIDON <fredbi@yahoo.com>

Author: fredbi

docs/doc-site/usage/USAGE.md

@@ -267,7 +267,7 @@ When unsure about argument order:
 
 ### Forward Methods
 
-Create an `Assertion` object to reduce repetition in tests with many assertions:
+Create an `Assertions` object to reduce repetition in tests with many assertions:
 
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