Fix/improve documentation of attr-lowercase rule (#564)

The "violation" example was identical to the non-violation one.

Also adjusted wording and capitalization for clarity and correctness.

docs/user-guide/rules/attr-lowercase.md

@@ -11,18 +11,18 @@ Level: `error`
 
 1. true: enable rule
 2. false: disable rule
-3. ['viewBox', 'test']: Ignore some attr name
+3. ['viewBox', 'Test']: enable rule except for the given attribute names
 
-The following pattern are **not** considered violations:
+The following pattern is **not** considered a violation:
 
 <!-- prettier-ignore -->
 ```html
 <img src="test.png" alt="test" />
 ```
 
-The following pattern is considered violation:
+The following pattern is considered a violation:
 
 <!-- prettier-ignore -->
 ```html
-<img src="test.png" alt="test" />
+<img SRC="test.png" ALT="test" />
 ```