chore: regenerate README (#422)

This PR was generated using Autosynth. 🌈


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

```
2021-05-13 14:02:50,568 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-datastore/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working tree clean
2021-05-13 14:02:51,812 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/cec070fa-bb3b-4290-bc82-9829f3cb2b09/targets

- [ ] To automatically regenerate this PR, check this box. (May take up to 24 hours.)

Author: yoshi-automation

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-datastore.git",
-        "sha": "9e943c1cc4722d7e8ef75c5c726975ef19f11fea"
+        "sha": "a1ea05f975365a7eba6607aeae1d144b8c49453c"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "f7e7449a641ce47f2a04ed577dec48e109325e76"
+        "sha": "bd8281a06cc7f84906e04d4843c1d3d386a980cd"
       }
     }
   ]

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>20.2.0</version>
+      <version>20.3.0</version>
       <type>pom</type>
       <scope>import</scope>
     </dependency>