[MINOR] Revise SparkSession documentation

### What changes were proposed in this pull request?

This PR aims to revise `SparkSession` documentation by fixing typos and inconsistency.

### Why are the changes needed?

To provider a better document.

### Does this PR introduce _any_ user-facing change?

No behavior change.

### How was this patch tested?

Manual review.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes #162 from dongjoon-hyun/minor_doc.

Authored-by: Dongjoon Hyun <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: dongjoon-hyun

Sources/SparkConnect/Documentation.docc/GettingStarted.md

@@ -36,10 +36,11 @@ targets: [
 ```swift
 import SparkConnect
 
-let spark = SparkSession.builder()
+let spark = try await SparkSession
+    .builder
     .appName("MySwiftApp")
     .remote("sc://localhost:15002")
-    .build()
+    .getOrCreate()
 ```
 
 ### 2. DataFrame Operations

Sources/SparkConnect/Documentation.docc/SparkSession.md

@@ -9,10 +9,11 @@ The entry point for SparkConnect functionality.
 ### Creating a SparkSession
 
 ```swift
-let spark = SparkSession.builder()
-    .appName("My Swift Spark App")
+let spark = try await SparkSession
+    .builder
+    .appName("MySwiftApp")
     .remote("sc://localhost:15002")
-    .build()
+    .getOrCreate()
 ```
 
 ### Basic Usage