docs(schema): minor fixes to existing YAML examples

Author: hassankhan

docs/content/Schema/Fundamentals/Concepts.mdx

@@ -52,7 +52,7 @@ cube('Orders', {
 ```yaml
 cubes:
   - name: Orders
-    sql: 'SELECT * FROM orders'
+    sql: SELECT * FROM orders
 ```
 
 </CodeTabs>

docs/content/Schema/Reference/cube.mdx

@@ -163,7 +163,7 @@ cubes:
         type: string
 
   - name: Contacts
-    sql: SELECT * FROM CONTACTS
+    sql: SELECT * FROM contacts
     dimensions:
       - name: id
         sql: id

docs/content/Schema/Reference/joins.mdx

@@ -86,7 +86,7 @@ cubes:
     joins:
       - name: Products
         relationship: belongs_to
-        sql: '{CUBE.id} = {Products.order_id}'
+        sql: "{CUBE.id} = {Products.order_id}"
 ```
 
 </CodeTabs>
@@ -140,7 +140,7 @@ cubes:
     joins:
       - name: Profile
         relationship: has_one
-        sql: '{Users}.id = {Profile.user_id}'
+        sql: "{Users}.id = {Profile.user_id}"
 ```
 
 </CodeTabs>

docs/content/Schema/Reference/types-and-formats.mdx

@@ -311,7 +311,7 @@ cubes:
   - name: Orders
     measures:
       - name: revenue
-        sql: '{chargesAmount}'
+        sql: "{chargesAmount}"
         type: sum
 ```
 
@@ -392,7 +392,7 @@ cubes:
   - name: Orders
     measures:
       - name: avg_transaction
-        sql: ${transaction_amount}
+        sql: "{transaction_amount}"
         type: avg
 ```
 
@@ -511,7 +511,7 @@ cubes:
   - name: Orders
     measures:
       - name: purchase_conversion
-        sql: ${purchase}/${checkout}*100.0
+        sql: "{purchase}/{checkout}*100.0"
         type: number
         format: percent
 ```
@@ -706,9 +706,9 @@ cubes:
       - name: location
         type: geo
         latitude:
-          sql: '{CUBE}.latitude'
+          sql: "{CUBE}.latitude"
         longitude:
-          sql: '{CUBE}.longitude'
+          sql: "{CUBE}.longitude"
 ```
 
 </CodeTabs>