chore: regenerate README (#528)

This PR was generated using Autosynth. 🌈


<details><summary>Log from Synthtool</summary>

```
2021-05-17 01:51:39,836 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-dlp/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-05-17 01:51:41,164 synthtool [DEBUG] > Wrote metadata to .github/readme/synth.metadata/synth.metadata.

```
</details>

Full log will be available here:
https://source.cloud.google.com/results/invocations/7b8656d7-d611-4fbd-8636-e815b08c9677/targets

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Author: yoshi-automation

.github/readme/synth.metadata/synth.metadata

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-dlp.git",
-        "sha": "fcb4e406b67d0a3ca9fc667f9941f1c38ba52712"
+        "sha": "1e10cd9ab4ea7e47fcb6a395569000e14d7035ea"
       }
     },
     {

README.md

@@ -17,7 +17,7 @@ If you are using Maven with [BOM][libraries-bom], add this to your pom.xml file
     <dependency>
       <groupId>com.google.cloud</groupId>
       <artifactId>libraries-bom</artifactId>
-      <version>20.3.0</version>
+      <version>20.4.0</version>
       <type>pom</type>
       <scope>import</scope>
     </dependency>
@@ -46,7 +46,7 @@ If you are using Maven without BOM, add this to your dependencies:
 
 If you are using Gradle 5.x or later, add this to your dependencies
 ```Groovy
-implementation platform('com.google.cloud:libraries-bom:20.3.0')
+implementation platform('com.google.cloud:libraries-bom:20.4.0')
 
 compile 'com.google.cloud:google-cloud-dlp'
 ```