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;
 ```