Update swift-markdown and fix test case (#708)

Author: Kyle-Ye

Package.resolved

@@ -177,9 +177,9 @@ class DocumentationContextTests: XCTestCase {
 │  │  │  │  ├─ Argument text segments:
 │  │  │  │  |    "source: figure1.png, alt: figure1 "
 │  │  │  ├─ Paragraph
-│  │  │  │  └─ Image source: "figure1" title: ""
+│  │  │  │  └─ Image source: "figure1"
 │  │  │  ├─ Paragraph
-│  │  │  │  └─ Image source: "images/figure1" title: ""
+│  │  │  │  └─ Image source: "images/figure1"
 │  │  │  └─ Paragraph
 │  │  │     └─ Text "Quis auctor elit sed vulputate mi sit amet."
 │  │  ├─ BlockDirective name: "Comment"
@@ -345,7 +345,7 @@ class DocumentationContextTests: XCTestCase {
 │  │  │  ├─ Paragraph
 │  │  │  │  └─ Text "Phasellus faucibus scelerisque eleifend donec pretium."
 │  │  │  ├─ Paragraph
-│  │  │  │  └─ Image source: "something.png" title: ""
+│  │  │  │  └─ Image source: "something.png"
 │  │  │  │     └─ Text "Diagram"
 │  │  │  ├─ CodeBlock language: swift
 │  │  │  │  let scene = ARSCNView()

Tests/SwiftDocCTests/Infrastructure/DocumentationContext/DocumentationContextTests.swift

@@ -154,7 +154,7 @@ class DocumentationMarkupTests: XCTestCase {
             Discussion content.
             """
             let expected = """
-            Image source: "image.jpg" title: ""
+            Image source: "image.jpg"
             └─ Text "Image title"
             Text " Abstract."
             """

Tests/SwiftDocCTests/Model/DocumentationMarkupTests.swift

@@ -46,7 +46,7 @@ class RenderContentCompilerTests: XCTestCase {
         ├─ Paragraph
         │  └─ Link destination: "doc:article2"
         │     ├─ Text "Custom Image Content "
-        │     └─ Image source: "https://example.com/test.png" title: ""
+        │     └─ Image source: "https://example.com/test.png"
         │        └─ Text "random image"
         ├─ Paragraph
         │  └─ Link destination: "doc:UNRESOVLED"