Update 103-avg.md

Added SQL language identifier to enable syntax highlighting

src/data/roadmaps/sql/content/104-aggregate-queries/103-avg.md

@@ -3,7 +3,7 @@
 The `AVG()` function in SQL is an aggregate function that returns the average value of a numeric column. It calculates the sum of values in a column and then divides it by the count of those values.
 
 Syntax:
-```
+```sql
 SELECT AVG(column_name)
 FROM table_name;
 ```
@@ -37,4 +37,4 @@ GROUP BY CustomerID;
 ```
 It will calculate the average quantity for each customer and display the result along with the associated customer's ID.
 
-> Note: The `AVG()` function works only with numeric data types (`INT`, `FLOAT`, `DECIMAL`, etc.). It will return an error if used with non-numeric data types.
+> Note: The `AVG()` function works only with numeric data types (`INT`, `FLOAT`, `DECIMAL`, etc.). It will return an error if used with non-numeric data types.