Update doc generation script (#335)

Motivation:

The docs haven't been generated for a while.

See also: apple/swift-nio#2013

Modifications:

- Pass `--spm` flag to sourcekitten
- Update source location of sourcekitten
- Fix link to NIO docs

Result:

Docs build.

scripts/generate_docs.sh

@@ -26,20 +26,20 @@ if [[ "$(uname -s)" == "Linux" ]]; then
     swift build
   fi
   # setup source-kitten if required
-  source_kitten_source_path="$root_path/.SourceKitten"
+  mkdir -p "$root_path/.build/sourcekitten"
+  source_kitten_source_path="$root_path/.build/sourcekitten/source"
   if [[ ! -d "$source_kitten_source_path" ]]; then
     git clone https://github.com/jpsim/SourceKitten.git "$source_kitten_source_path"
   fi
-  source_kitten_path="$source_kitten_source_path/.build/x86_64-unknown-linux/debug"
+  source_kitten_path="$source_kitten_source_path/.build/debug"
   if [[ ! -d "$source_kitten_path" ]]; then
     rm -rf "$source_kitten_source_path/.swift-version"
     cd "$source_kitten_source_path" && swift build && cd "$root_path"
   fi
   # generate
-  mkdir -p "$root_path/.build/sourcekitten"
   for module in "${modules[@]}"; do
     if [[ ! -f "$root_path/.build/sourcekitten/$module.json" ]]; then
-      "$source_kitten_path/sourcekitten" doc --module-name $module > "$root_path/.build/sourcekitten/$module.json"
+      "$source_kitten_path/sourcekitten" doc --spm --module-name $module > "$root_path/.build/sourcekitten/$module.json"
     fi
   done
 fi
@@ -81,7 +81,7 @@ To secure a server connection, you will need a X.509 certificate chain in a file
 
 For the API documentation of the other repositories in the SwiftNIO family check:
 
-- [`swift-nio` API docs](https://apple.github.io/swift-nio/docs/current/NIO/index.html)
+- [`swift-nio` API docs](https://apple.github.io/swift-nio/docs/current/NIOCore/index.html)
 - [`swift-nio-ssl` API docs](https://apple.github.io/swift-nio-ssl/docs/current/NIOSSL/index.html)
 - [`swift-nio-http2` API docs](https://apple.github.io/swift-nio-http2/docs/current/NIOHTTP2/index.html)
 - [`swift-nio-extras` API docs](https://apple.github.io/swift-nio-extras/docs/current/NIOExtras/index.html)