Fix attribute grammar and update documentation

GRAMMAR.md

@@ -124,9 +124,11 @@ sequence      : expression ',' sequence
 
 recipe        : attributes* '@'? NAME parameter* variadic? ':' dependencies eol body?
 
-attributes    : '[' attribute* ']' eol
+attributes    : '[' attribute (',' attribute)* ']' eol
 
-attribute     : NAME ( '(' string ')' )?
+attribute     : NAME
+              | NAME ':' string
+              | NAME '(' string (',' string)* ')'
 
 parameter     : '$'? NAME
               | '$'? NAME '=' value

README.md

@@ -2108,6 +2108,13 @@ foo:
     echo "foo"
 ```
 
+Attributes with a single argument may be written with a colon:
+
+```just
+[group: 'bar']
+foo:
+```
+
 #### Enabling and Disabling Recipes<sup>1.8.0</sup>
 
 The `[linux]`, `[macos]`, `[unix]`, and `[windows]` attributes are