Fix typo "on(e) category" to "a single category" in user docs

Co-authored-by: Sebastian Fey <sepp.fey@web.de>
Signed-off-by: Christian Wolf <github@christianwolf.email>

docs/user/index.md

@@ -31,7 +31,7 @@ At the moment it is not possible to share a public link to a recipe.
 
 The use of keywords and categories is entirely up to you.
 
-The primary difference between the two is that a recipe can only have on category,
+The primary difference between the two is that a recipe can only have a single category,
 but may have many keywords.
 In other words,
 categories are a 1:N relationship while keywords are an N:M relationship.