chore: regenerate README (#555)

This PR was generated using Autosynth. 🌈


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

```
2020-10-26 23:59:21,282 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-spanner/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working directory clean
2020-10-26 23:59:22,319 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/d2bdcfe4-2eeb-4ec5-9c45-4c5f2d939447/targets

- [ ] To automatically regenerate this PR, check this box.

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-spanner.git",
-        "sha": "edb7ef41c599b71f3b57e429189f70db5f72b77b"
+        "sha": "226c4f52aa252b9655ad39bf7825b490cf6de782"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "a783321fd55f010709294455584a553f4b24b944"
+        "sha": "fd3584b01cedd6c9f79d08be4e2365085f955aa5"
       }
     }
   ]

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>13.2.0</version>
+      <version>13.3.0</version>
       <type>pom</type>
       <scope>import</scope>
     </dependency>