Add javadoc step to GH Actions

.github/workflows/build.yml

@@ -17,20 +17,22 @@ jobs:
             fail-fast: true
         steps:
             - uses: actions/checkout@v2
-            - name: JDK ${{ matrix.java }}
+            - name: JDK ${{ matrix.java }}
               uses: actions/setup-java@v1
               with:
-                  java-version: ${{ matrix.java }}
+                  java-version: ${{ matrix.java }}
             - name: Cache maven
               uses: actions/cache@v2
               with:
                   path: |
                       ~/.m2/repository/
                       work/Minecraft
-                  key: ${{ runner.os }}-paper-${{ hashFiles('**/pom.xml') }}
+                  key: ${{ runner.os }}-paper-${{ hashFiles('**/pom.xml') }}
                   restore-keys: ${{ runner.os }}-paper
             - name: Patch and build
               run: |
                   git config --global user.email "no-reply@github.com"
                   git config --global user.name "Github Actions"
                   ./paper build
+            - name: Build javadocs
+              run: mvn -pl Paper-API,Paper-MojangAPI -am javadoc:javadoc