Fix typo "a UnknownSyntax" -> "an UnknownSyntax"

Author: ahoppen

Sources/SwiftSyntax/SyntaxNodes.swift.gyb

@@ -53,7 +53,7 @@ extension Syntax {
 public struct UnknownSyntax: SyntaxProtocol {
   public let _syntaxNode: Syntax
 
-  /// Convert the given `Syntax` node to a `UnknownSyntax` if possible. Return 
+  /// Convert the given `Syntax` node to an `UnknownSyntax` if possible. Return 
   /// `nil` if the conversion is not possible.
   public init?(_ syntax: Syntax) {
     guard syntax.raw.kind == .unknown else { return nil }

Sources/SwiftSyntax/SyntaxRewriter.swift.gyb

@@ -47,7 +47,7 @@ open class SyntaxRewriter {
     return Syntax(token)
   }
 
-  /// Visit a `UnknownSyntax`.
+  /// Visit an `UnknownSyntax`.
   ///   - Parameter node: the node that is being visited
   ///   - Returns: the rewritten node
   open func visit(_ node: UnknownSyntax) -> Syntax {