[css-mixins-1] Fix space placement in serialize a CSSFunctionRule (#13605)

* [css-mixins-1] Fix numbering of `serialize a CSSFunctionRule`

* [css-mixins-1] Fix space placement in `serialize a CSSFunctionRule`

By moving the space from step 5 to the start of step 6 we ensure that
there is a space before the '{' regardless of whether we serialize the
return type (i.e. "--foo() {..." rather than "--foo(){...")

By moving the space from after the '{' in step 6 to before the first
serialized rule in step 7 we ensure that there is only one space (the
one in step 8) between the '{' and '}' in the case that there are no
rules.

Author: Calme1709

css-mixins-1/Overview.bs

@@ -2353,33 +2353,32 @@ return the concatenation of the following:
 	2. The result of performing <a>serialize an identifier</a>
 		on the name of the [=custom function=],
 		followed by a single LEFT PARENTHESIS (U+0028).
-	4. The result of [=serialize a function parameter=]
+	3. The result of [=serialize a function parameter=]
 		on each of the [=custom function's=] [=function parameter|parameters=],
 		all joined by <code>", "</code>
 		(COMMA U+002C, followed by a single SPACE U+0020).
-	5. A single RIGHT PARENTHESIS (U+0029).
-	6. If the [=custom function=] has [=custom function/return type=],
+	4. A single RIGHT PARENTHESIS (U+0029).
+	5. If the [=custom function=] has [=custom function/return type=],
 		and that [=custom function/return type=]
 		is not the [=universal syntax definition=] ("*"):
 		* A single SPACE (U+0020),
 			followed by the string <code>"returns"</code>,
 			followed by a single SPACE (U+0020).
 		* The result of performing [=serialize a CSS type=]
-			on that [=custom function/return type|type=],
-			followed by a single SPACE (U+0020).
+			on that [=custom function/return type|type=].
 
-	7. A single LEFT CURLY BRACKET (U+007B),
-		followed by a SPACE (U+0020).
+	6. A single SPACE (U+0020),
+		followed by a LEFT CURLY BRACKET (U+007B).
 
-	8. The result of performing [=serialize a CSS rule=]
+	7. The result of performing [=serialize a CSS rule=]
 		on each rule in cssRules,
 		filtering out empty strings,
-		all joined by a single SPACE (U+0020).
+		each preceded by a single SPACE (U+0020).
 
 		Note: [=Serialize a CSS rule=] can return an empty string
 			when serializing an empty {{CSSFunctionDeclarations}} rule.
 
-	9. A single SPACE (U+0020),
+	8. A single SPACE (U+0020),
 		followed by a single RIGHT CURLY BRACKET (U+007D).
 </div>